fix: disable dry-run by default

Attempting to specify `#[clap(default_value_t = false)] or other
variations resulted in this value always being true. Removing the value
causes the flag to default to false and be set to true if `--dry-run` is
included.

src/cli.rs

@@ -6,6 +6,5 @@ pub struct Args {
     pub old_version: String,
     pub new_version: String,
 
-    #[clap(short, long, default_value = "false")]
     pub dry_run: bool,
 }