Do not build Clippy by default

[mati865]

For rust-lang/rust#59761

As the bonus I changed Travis to test Clippy only when commit message contains words update, clippy. You can see it in the action here: https://travis-ci.com/mati865/rls/builds/114715073 https://travis-ci.com/mati865/rls/builds/114715514
That trick was borrowed from Rust: https://github.com/rust-lang/rust/blob/ca1bcfdde3f19afd68ef808cecf2ce56d08d5df4/.travis.yml#L218

[Xanewok]

(...) only when commit message contains words update

That does seem fragile - ideally we'd rebuild whenever resolved clippy_lints rev changes in Cargo.toml but let's land that as-is

[mati865]

ideally we'd rebuild whenever resolved clippy_lints rev changes in Cargo.toml

With some trickery involved it'd possible to make Clippy job always run and return early if clippy_lints line hasn't changed.

[mati865]

Rebased on 7ec4b12

[alexheretic]

Good idea! Clippy is a real pain for rls development/ci at the moment.