Pretty Travis cfg

[kpp]

See https://travis-ci.com/async-rs/async-std/builds/124829949 for nicely separated build jobs.

[skade]

I'd still prefer environment config to be per matrix (to avoid the nasty "overwrite env" behaviour) or be moved into a build script in the ci folder, but otherwise, this looks good.

[kpp]

I'd still prefer environment config to be per matrix

Since I eliminated env vars BUILD_DOCS and BUILD_BOOK you don't need to set up env var per matrix. Every matrix build get pure RUSTFLAGS="-D warnings". I can put it down explicitly if you want but there is no difference at all. Shall I put it down?

[kpp]

@skade ping?

[kpp]

@yoshuawuyts @stjepang r?

[ghost]

I like this a lot. Thanks!