[TEMP] Disable caching bower packages and registry

Disable caching bower packages and registry temporarily.
This is a workaround for https://github.com/bower/bower/issues/933.

Use this branch, if you faced a heroku deployment error looks like this:
```
       bower jquery#>= 1.7.1        ENOTEMPTY ENOTEMPTY, rename 'vendor/bower/tmp/jquery-1435-FddgpL'
       Stack trace:
       Error: ENOTEMPTY, rename 'vendor/bower/tmp/jquery-1435-FddgpL'
```

How to use:
```shell
$ heroku config:set -a YOUR_APP_NAME \
  BUILDPACK_URL='git://github.com/qnyp/heroku-buildpack-ruby-bower.git#workaround-for-bower-cache-bug'
```

I'll keep this branch till bower fixes problem.