Reset Ember dependency to rc.6
Reset Ember dependency to rc.6

According to [the Ember components repository](https://github.com/components/ember), which is where Bower looks for Ember, it hasn't updated to rc.6.1 yet. Resetting this so those getting started with EAK don't have to troubleshoot this error until components updates.
achambers/appkit-hanging-test-example,arjand/weddingapp,tobobo/shibe_io_frontend,kiwiupover/ember-weather,daevid/broccoli-todo,mtian/ember-app-kit-azure,aboveproperty/ember-app-kit-karma,grese/miner-app-OLD,stefanpenner/ember-app-kit,Anshdesire/ember-app-kit,mixonic/ember-app-kit,kiwiupover/ember-weather,lucaspottersky/ember-lab-eak,tucsonlabs/ember-weather-legacy,cevn/tasky-web,tobobo/vice-frontend,teddyzeenny/assistant,lucaspottersky/ember-lab-eak,thaume/emberjs-es6-modules,tomclose/minimal_eak_test_stub_problem,shin1ohno/want-js,digitalplaywright/eak-simple-auth-blog-client,tobobo/thyme-frontend,duvillierA/try_ember,ClockworkNet/ember-boilerplate,grese/miner-app-OLD,sajt/ember-weather,Anshdesire/ember-app-kit,ghempton/ember-libs-example,kiwiupover/seattle-auckland-weather,sajt/ember-weather,mtian/ember-app-kit-azure,mharris717/eak_base_app,joliss/broccoli-todo,digitalplaywright/eak-simple-auth,grese/btcArbitrage,shin1ohno/EmberAppKit,eriktrom/ember-weather-traininig,tomclose/test_loading_problem,stefanpenner/ember-app-kit,trombom/ember-weather-traininig,ClockworkNet/ember-boilerplate,tobobo/thyme-frontend,jwlms/ember-weather-legacy,michaelorionmcmanus/eak-bootstrap,ebryn/ember-github-issues