Consider switching CI to GitHub Actions

[Xanewok]

Right now we have

• AppVeyor builds for testing {x86_64, i686}-pc-windows-msvc and
• Travis CI for x86_64-unknown-linux-gnu

which requires maintaining two config files and integrations; I also vaguely recall some problems when using AppVeyor, e.g. now it fails on missing rust-src in https://ci.appveyor.com/project/TedDriggs/racer-xr5g5/builds/36589888/job/fj30kkmy8hx7053c but Travis CI works without problems.

@kngwyu Would you accept a PR that sets up GitHub Actions for both of these platforms? We could also schedule nightly builds to catch breakage early, e.g. see rust-lang/rust-semverver@80fac90 what's done in rust-semverver.