Make the build fail on warnings during testing on Travis

[thomaseizinger]

With the merge of #300, we have a lot of warnings in our code base.

These should obviously be fixed.

Once that is done, we should find a way to fail the CI build on warnings.

[thomaseizinger]

This code shows how this can be done with clippy: https://github.com/rust-lang-nursery/rust-clippy#travis-ci

Open questions:

• Does clippy also emit warnings generated by rustc? If yes, then we can simply use clippy to fail the build upon any warnings.

[thomaseizinger]

cargo fix will eventually be able to fix clippy warnings, we can thus ignore them for now and fix them automatically later.

Source: rust-lang/rustfix#130

[D4nte]

Done!