Readme refers to new 'ember-source', '1.0.0.rc6.1'

I pushed [http://rubygems.org/gems/ember-source/versions/1.0.0.rc6.1](ember-source rc6.1) to Rubygems, removing its dependency on pre-release Handlebars, which means we are free from a chunk of pre-release hell. Specifically, if you want to use a non-pre-release Handlebars (i.e. >= 1.0.0), you don't have to specify `handlebars-source` in your Gemfile anymore. Yay.