Add integration tests

[gnzlbg]

Cargo's CI should verify that updates to cargo are still able to properly compile a collection of already existing crates without issues.

For inspiration, rustfmt uses many crates in the nursery as its "integration test suite":

• https://github.com/rust-lang-nursery/rustfmt/blob/master/.travis.yml#L21
• https://github.com/rust-lang-nursery/rustfmt/blob/master/ci/integration.sh

[ehuss]

An update on this: as of rust-lang/rust#73267, we are now testing in rust-lang/rust's CI for a small set of packages (iron, ripgrep, tokei, treeify, xsv, servo)

[epage]

Sounds like this is resolved, even if delayed until rust-lang/rust updates the copy of cargo. Considering the impact of CI time on development, this seems like a reasonable trade off.