Lock proc-macro2 to last working version

proc-macro2 breaks genco (see
udoprog/genco#39). For now, we can introduce a
version cap like this, so that it works even when someone installs
without `--locked`.

Author: obrok

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Invalid version of an indirect dependency causing mangled output when installed without `--locked`
+
 ## 0.4.1
 
 ### Fixed