Simplify `.travis.yml`

Travis already prints the versions of `rustc` and `cargo` right after it
installs them. Also, the default test script already runs verbose `cargo build`
and `cargo test`, so really all that's needed here is to ensure that
`cargo doc` is executed after the script (`after_script`).
Merge pull request #3 from KenanY/simplify-travis

Simplify `.travis.yml`