Mark ember-release as may-fail

The file ember-release changes over time, which makes it difficult
to patch old versions of Ember-I18n. This commit

 * removes the ember-release tests from `make test_stables`
 * adds a test against ember-1.9.1 to `make test_stables`
 * adds a test against ember-release to `make test_prereleases`
 * adds the ember-release test to the may-fail list in `.travis.yml`

In summary, it adds must-pass tests against ember-1.9.1 (a locked
version) and makes the tests against ember-release (a dynamic version)
may-fail.