Misc travis-ci build tweaks

* Add faraday_middleware gem to address travis-artifacts error:
    /home/travis/.rvm/gems/ruby-1.9.3-p484/gems/faraday-0.9.0.rc7/lib/faraday.rb:189:in `lookup_middleware': :json is not registered on Faraday::Response (Faraday::Error)
* Move gem install to after_success so builds that fail, fail faster
* Make PROJECT_VERSION available to all build stages