Added support for downloading a tagged version of ember or ember-data

While I was at it, I added support for downloading a specific tagged release. The syntax is:
```sh
rails g ember:install --tag=v1.0.0-beta.3/ember-data
````
when you want to download a specific tagged ember-data version

```sh
rails g ember:install --tag=v1.0.0-rc.8/ember
````
when you want to download a tagged specific ember version

Also

- fixed a typo in the help text
- Now rightfully downloading the min file for production instead of the debug version