Force rubyzip 0.9.7 (0.9.9 breaks travis ?)

* ref https://travis-ci.org/petervandenabeele/dbd_data_engine/jobs/10812547
* http://jayhsunote.blogspot.be/2013/08/ruby-on-rails-excel-gem-roo-loaderror.html (yesterday)
* neography only needs 0.9.7
* error was /home/travis/.rvm/gems/ruby-2.0.0-p247/gems/neography-1.1.2/lib/neography.rb:8:in `require': cannot load such file -- zip/zipfilesystem (LoadError)