compiletest failures don't fail CI

[jonas-schievink]

The compiletest suite requires nightly, but Travis is configured to allow_failures on nightly. They are essential to test soundness, so this is not good.

[jonas-schievink]

Luckily the only failures I see are due to wording changes in error messages

[jonas-schievink]

It might make sense to move these tests over to trybuild and run them only on the current stable instead of nightly.

[jonas-schievink]

It looks like trybuild will only run cargo check, and can not be coerced to actually codegen and link, which is required by some tests to detect duplicate symbols. Luckily compiletest should now also run on stable though.