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

@rlivsey @eventualbuddha @ebryn @hjdivad @locks @pjmorse

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

Closes #153