.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.
Use uglify-js, not uglify

The former seems to be a much better maintained project.
Merge pull request #183 from jamesarosen/james/allow-prerelease-failures

.travis.yml: tests against Ember-beta and -canary may fail
Drop support for {{translateAttr}}

Dropping for a variety of reasons:

 * It doesn't  support bound values, which confuses users
 * Users can achieve the same effect with `TranslateableAttributes`
   or by creating translated properties on the controller or
   component
 * It doesn't play nicely with the current HTMLbars-Handlebars
   compatibility layer

See https://github.com/emberjs/ember.js/issues/9961