Add param to set bundler flags

Using a custom omniauth provider often requires adding an additional gem.  To install gems with a modified gem file, the bundle command must be run with the ```--no-deployment``` flag.  See [Custom omniauth provider configurations](https://github.com/gitlabhq/gitlab-public-wiki/wiki/Custom-omniauth-provider-configurations) for the recommendation.

This is probably the first step in allowing configuration of omniauth.