Bump version to `2.1` because of `bind-attr` syntax change.

In recent version of Ember.js the bound attribute syntax has changed
from `<div {{bindAttr foo=bar}}>` to `<div {{bind-attr foo=bar}}>`. We
have bumped to 2.1 to stop people being surprised by this on older Ember
apps.
Let's test on current stable RBX.