Consider setting up continuous integration to check pull requests against rustc 1.24.1

[dtolnay]

For example a minimal .travis.yml for this would be:

language: rust
rust: 1.24.1
script: cargo check

This would avoid the problem described in #2 (comment) of pull requests breaking compatibility with rustc versions that need to be supported.

[otavio]

@mmstick which is the oldest supported version?

[mmstick]

Unsure what the oldest supported version of the crate is, but I added a toolchain file to limit it to Rust 1.35.0, because that's what Ubuntu Bionic currently support.

[otavio]

The 1.24.1 works just fine, I'd avoid pining a new version without a need.