[BUGFIX] Register a default RSVP error handler.

This will significantly improve the users experience when debugging
errors thrown in promises as they will now be logged to the console in
development.
Merge pull request #3743 from rjackson/log_promise_errors

[BUGFIX] Register a default RSVP error handler.
[BUGFIX release] Update to Handlebars 1.1.2.

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).
Merge pull request #3799 from rjackson/update_handlebars_version

[BUGFIX release] Update to Handlebars 1.1.2.