Remove unnecessary .gitmodules

Currently, Ember.js is bundled as ember-source gem.
Merge pull request #390 from tricknotes/remove-gitmodules

Remove unnecessary .gitmodules
Suppress deprecation warning for appraisal

```
`rake appraisal:install` task is deprecated and will be removed soon.
Please use `appraisal install`.
```
ember-source has been updated on October 28th 2014

Update the suggested version of ember-source to 1.8.0
Merge pull request #394 from jure/update_ember_source

Update the README suggested version of ember-source to 1.8.0
bumping version on readme
Merge pull request #399 from thejamespinto/patch-1

bumping version on readme
Test against Rails 4.2.0.rc1
Update description of generator option in README.md

`--javascript-engine` allows `em`.
Merge pull request #402 from tricknotes/readme

Update description of generator option in README.md
Fix tests for Handlebars 2.0
Merge pull request #405 from tricknotes/fix-tests-for-hbs-2.0

Fix tests for Handlebars 2.0
Fix generator for `--channel=release`

Default channel is `:release` (Symbol) but channel from option is
`"release"` (String).
So `rails g ember:install --channel=release` couldn't fallback to beta channel
and it causes download error:
```
ERROR: Error reading the content from the channel with url
http://builds.emberjs.com/release/ember-data.js. File not found
```
Use ember.debug.js if it exist

`ember.js` is deprecated in favor of `ember.debug.js`
later than Ember.js 1.10.0.beta.1.
* https://github.com/emberjs/ember.js/issues/9830
* https://github.com/emberjs/ember.js/pull/9845
Support ember.debug.js for generator
Merge pull request #404 from tricknotes/fix-release-channel-for-ember-data

Fix generator for `--channel=release`
Merge pull request #403 from tricknotes/ember.debug.js

Use ember.debug.js if it exist
Fix broken generator for ember-data

The channel fallback for ember-data doesn't work correctly.
`release -> beta` is expected but actual `release -> release`.

This cause is that the following PRs have incompatible changes:
* https://github.com/emberjs/ember-rails/pull/403
* https://github.com/emberjs/ember-rails/pull/404
Merge pull request #407 from tricknotes/fix-broken-generator

Fix broken generator for ember-data
Merge pull request #401 from tricknotes/rails-42

Test against Rails 4.2.0.rc1