Fix Rust 1.24 support by locking down some versions

[palfrey]

Based off the versions that worked in the last good 1.24 build at https://travis-ci.org/BurntSushi/fst/jobs/468402724

[BurntSushi]

Could you say why you are submitting this PR? The fst binary is more of a debugging tool and I don't really have any desire for maintaining support for it on versions of Rust that were released ~2 years ago.

FWIW, I am working on a new release of fst that bumps the MSRV to at least Rust 1.39 and will be hopefully putting that out soon.

[palfrey]

Mainly because I've been digging through the tools in awesome-rust that have broken builds and seeing what I can do to fix them TBH. If you've got no interest in this, then no worries, but I figured even after seeing the new version work it's useful as an interim measure maybe?

[BurntSushi]

Ah okay, just bad timing then. #96 will resolve this, and it also moves CI to GitHub Actions. Resolving this in the interim isn't that important. Thanks though!