Re-enable build on Rust Nightly on Travis CI.

[briansmith]

The builds are currently failing because of rust-lang/rust#30713 and so Nightly builds are temporarily being marked allow_failures until the builds are working again.

[briansmith]

Depends on rust-lang/rust#30733 landing.

[briansmith]

This was fixed in 2750be3 and fa01e36.

[briansmith]

8aa601e moved ring back to using a real release of num.