Ember-I18n 2.x is not compatible with Ember >= 1.9

Ember-I18n 2.x relies on private Ember.Handlebars APIs that
are being removed in Ember 1.9. Ember Canary is now what will
become Ember 1.9. Thus, we

 1. note the version restriction in bower.json
 2. note the version restriction in the README
 3. remove canary builds from Makefile
 4. remove canary builds from .travis.yml
 5. no longer vetch canary Ember as part of vendor dependencies

cf https://github.com/jamesarosen/ember-i18n/issues/153

Closes #153