Fixing ruby heroku buildpack

The build pack release step for the heroku-buildpack-ruby is currently broken. It fails to generate a .release file which  manifests as the following exception when attempting to YAML load the release file.

<pre>
<code>
/usr/lib/ruby/1.9.1/psych.rb:297:in `initialize': no implicit conversion from nil to integer (TypeError)
from /usr/lib/ruby/1.9.1/psych.rb:297:in `open'
from /usr/lib/ruby/1.9.1/psych.rb:297:in `load_file'
</code>
</pre>

Looking at the [heroku buildpack documentation](https://devcenter.heroku.com/articles/buildpack-api) I believe this is the correct way to invoke release (bin/release $BUILD_DIR)