Fix for issue #1380

Since devise is not listed as a gem dependency, rails_admin will have problems installing when it's missing from the gemset.

Steps to reproduce issue:

    gem uninstall devise
    rails new Issue1380
    cd Issue1380
    echo 'gem "rails_admin"' >> Gemfile
    bundle install
    rails g rails_admin:install


This patch calls `bundle install` after devise is appended to the Gemfile. It will make the install process slower when devise is not listed, but results in a more stable install environment.