Fix travisci config to avoid errors related to an OSX Gemfile.lock

An empty bundler_args removes the "--deployment" flag which breaks in
our case since I develop on OSX and Gemfile.lock hence contains
references to gems that are not the right ones on travisci boxes.

See: https://groups.google.com/forum/#!topic/travis-ci/XID21_FwgX8
See: http://docs.travis-ci.com/user/languages/ruby/