remove duplicate dependency on jquery-rails

After adding ember-appkit-rails to my Gemfile and running bundle install, I kept getting this warning:
```
ember-appkit-rails at /Users/frabrunelle/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/bundler/gems/ember-appkit-rails-9b18b19cbc22 did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  duplicate dependency on jquery-rails (>= 0, development), (>= 0) use:
    add_runtime_dependency 'jquery-rails', '>= 0', '>= 0'
```