Update Travis CI Rust version to Nightly 2015-05-01
Update Travis CI Rust version to Nightly 2015-05-01

I *believe* this should work: Travis CI seems to use `rustup.sh` to
install the required Rust version by specifing `--spec=%s` where %s
corresponst to the provided rust version. Using `rustup.sh
--spec=nightly-2015-05-01` seems to work locally, so it just might also
work on Travis CI.

Drakulix/zwreec,Drakulix/zwreec