chef-server Gemfile improvements

Currently chef-server-api's Gemfile references it's chef and chef-solr dependencies relatively which can be problematic when chef-server-api is run under Bundler outside of the Rubygems path.

Currently chef-server-webui has no real Gemfile and needs to be leveled u.p

I propose the following:

• make the chef and chef-solr normal Rubygem.org dependencies (Bundler's default behavior), the version for each should match the current chef-server-api's version
• add unicorn as a production dependency so the improved Gemfile can be easily leveraged in some forthcoming omnibus-chef improvements.
• add a grown-up Gemfile to chef-server-webui