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
```
Merge pull request #404 from tricknotes/fix-release-channel-for-ember-data

Fix generator for `--channel=release`