Update .travis.yml to test on all channels
Update .travis.yml to test on all channels

This updates .travis.yml to test each commit on stable, beta and nightly rust versions.
steveklabnik/ncollide,sebcrozet/ncollide,MichaelRiss/ncollide,MichaelRiss/ncollide,steveklabnik/ncollide,sebcrozet/ncollide