[travis] Drop allow_failures for nightly

[behnam]

This will increase the time waiting for CI tests to pass, but we care
about the nightly test to pass and features depending on nightly to be
stable. So, better just drop allow_failures.

[CAD97]

Useful side effect: we can add a rustfmt and/or clippy check on the nightly CI which would fail on bad formatting or common code smells, respectively. (Note rust-lang/rust-clippy#1707 for clippy)

[behnam]

rustfmt sounds good. clippy still needs some work to get there.

I think we can do these under a separate matrix config from normal nightly build, so we can easily see which part (build/tests, vs formatting) is making a problem.