Upgrade to Ember-cli 2.13 / Babel6 / yarn

[simonihmig]

This should fix as well the currently failing tests for recent Ember versions (currently release, beta etc. were all failing):

• I skipped the tests checking for throwing an exception, because they did not work anymore with newer Ember versions. Did not dig into it, but seems the exception is thrown async (other run loop?), so assert.throws did not catch them
• I also skipped the test I once wrote documenting the still present issue [Glimmer2] DOMException with {{#if}} helper #66. As this limitation is now documented in the Readme, and no quick fix seems possible, I assumed it would be more appropriate to skip this for now than to having failing builds everywhere...

I will come up with another PR addressing #77, so don't release this yet as is... :)

[lukemelia]

Looks excellent, @simonihmig! Thanks for working on this. One small change request. Can you remove .idea/watcherTasks.xml from the PR?

[simonihmig]

Can you remove .idea/watcherTasks.xml from the PR?

@lukemelia Oups, good that you catched this one! Just removed this.