Added Bundler.with_clean_env

Without `Bundler.with_clean_env`, and running `rake integrate`, gems referenced in github repositories, like `gem 'activeadmin', github: 'gregbell/active_admin'` are interpreted like rubygems dependency, and I receive an error:

<pre>
Bundler::GemNotFound: Could not find activeadmin-1.0.0.pre.gem for installation
An error occurred while installing activeadmin (1.0.0.pre), and Bundler cannot continue.
Make sure that `gem install activeadmin -v '1.0.0.pre'` succeeds before bundling.
</pre>

Because bundler tries to fetch from rubygems, instead of get it from activeadmin github repo, or retrive from the  local cache. This PR fix this behavior.