.travis.yml: tests against Ember-beta and -canary may fail

Ember is moving too rapidly for us to reliably test against the
beta and canary versions. It'll be nice to know when those builds
fail, but we can't let them stop us from working.
Merge pull request #183 from jamesarosen/james/allow-prerelease-failures

.travis.yml: tests against Ember-beta and -canary may fail