setup Bundler in engines `bin/rails` stub.

This is necessary when bundling gems locally using `BUNDLE_PATH`.
Without this patch `bin/rails` fails with:

```
/Users/senny/.rbenv/versions/2.0.0-p353/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- rails/all (LoadError)
	from /Users/senny/.rbenv/versions/2.0.0-p353/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from bin/rails:7:in `<main>'
```

I wasn't sure what's the best way to test this, without effectively calling bundle and using a `BUNDLE_PATH`, which would take some time.