[BUGFIX release] Update to Handlebars 1.1.2.

Handlebars 1.1.2 allows compiling empty templates (which doesn't work under 1.1.1).

Since this only affects the tooling, we don't need to republish a 1.2.1 for this, but I tagged it with `[BUGFIX release]` so that we can get the stable branch `.gemspec` updated. Once merged, I can publish an updated gem as 1.2.0.1.

Also, uses allows future point releases to be allowed for consumer projects (ember-rails, ember-appkit-rails, etc).