Simplify `.travis.yml`

Travis [already prints](https://travis-ci.org/andelf/rust-iconv/builds/39691496#L21) the versions of `rustc` and `cargo` right after it installs them. Also, the [default test script](http://docs.travis-ci.com/user/languages/rust/#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`).