mdBook fails cargo install due to handlebars change

[ehuss]

Because Cargo.lock is not published by default, running cargo install mdbook is now failing because it is picking up a new version of handlebars which is incompatible. See sunng87/handlebars-rust#249. I am getting these errors:

error[E0061]: this function takes 2 parameters but 3 parameters were supplied
  --> src/renderer/html_handlebars/helpers/navigation.rs:55:23
   |
55 |     let chapters = rc.evaluate_absolute(ctx, "chapters", true).and_then(|c| {
   |                       ^^^^^^^^^^^^^^^^^ expected 2 parameters

error[E0308]: mismatched types
  --> src/renderer/html_handlebars/helpers/navigation.rs:56:57
   |
56 |         serde_json::value::from_value::<Vec<StringMap>>(c.clone())
   |                                                         ^^^^^^^^^ expected enum `handlebars::Value`, found enum `std::option::Option`
   |
   = note: expected type `handlebars::Value`
              found type `std::option::Option<&handlebars::Value>`

error[E0061]: this function takes 2 parameters but 3 parameters were supplied
  --> src/renderer/html_handlebars/helpers/navigation.rs:61:10
   |
61 |         .evaluate_absolute(ctx, "path", true)?
   |          ^^^^^^^^^^^^^^^^^ expected 2 parameters

error[E0599]: no method named `as_str` found for type `std::option::Option<&handlebars::Value>` in the current scope
  --> src/renderer/html_handlebars/helpers/navigation.rs:62:10
   |
62 |         .as_str()
   |          ^^^^^^

error[E0277]: the size for values of type `str` cannot be known at compilation time
  --> src/renderer/html_handlebars/helpers/navigation.rs:60:9
   |
60 |     let base_path = rc
   |         ^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `str`
   = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
   = note: all local variables must have a statically known size
   = help: unsized locals are gated as an unstable feature

error[E0061]: this function takes 2 parameters but 3 parameters were supplied
   --> src/renderer/html_handlebars/helpers/navigation.rs:100:10
    |
100 |         .evaluate_absolute(ctx, "path", false)?
    |          ^^^^^^^^^^^^^^^^^ expected 2 parameters

error[E0599]: no method named `as_str` found for type `std::option::Option<&handlebars::Value>` in the current scope
   --> src/renderer/html_handlebars/helpers/navigation.rs:101:10
    |
101 |         .as_str()
    |          ^^^^^^

error[E0061]: this function takes 2 parameters but 3 parameters were supplied
  --> src/renderer/html_handlebars/helpers/theme.rs:20:10
   |
20 |         .evaluate_absolute(ctx, "default_theme", true)?
   |          ^^^^^^^^^^^^^^^^^ expected 2 parameters

error[E0599]: no method named `as_str` found for type `std::option::Option<&handlebars::Value>` in the current scope
  --> src/renderer/html_handlebars/helpers/theme.rs:21:10
   |
21 |         .as_str()
   |          ^^^^^^

error[E0061]: this function takes 2 parameters but 3 parameters were supplied
  --> src/renderer/html_handlebars/helpers/toc.rs:28:27
   |
28 |         let chapters = rc.evaluate_absolute(ctx, "chapters", true).and_then(|c| {
   |                           ^^^^^^^^^^^^^^^^^ expected 2 parameters

error[E0308]: mismatched types
  --> src/renderer/html_handlebars/helpers/toc.rs:29:76
   |
29 |             serde_json::value::from_value::<Vec<BTreeMap<String, String>>>(c.clone())
   |                                                                            ^^^^^^^^^ expected enum `handlebars::Value`, found enum `std::option::Option`
   |
   = note: expected type `handlebars::Value`
              found type `std::option::Option<&handlebars::Value>`

error[E0061]: this function takes 2 parameters but 3 parameters were supplied
  --> src/renderer/html_handlebars/helpers/toc.rs:33:14
   |
33 |             .evaluate_absolute(ctx, "path", true)?
   |              ^^^^^^^^^^^^^^^^^ expected 2 parameters

error[E0599]: no method named `as_str` found for type `std::option::Option<&handlebars::Value>` in the current scope
  --> src/renderer/html_handlebars/helpers/toc.rs:34:14
   |
34 |             .as_str()
   |              ^^^^^^

error: aborting due to 13 previous errors

[ehuss]

Handlebars 1.2 has been yanked.