Remove ruby-head from .travis.yml

* always getting (in all projects) :

  $ gem query --local | grep bundler >/dev/null || gem install bundler

  ERROR: Could not find a valid gem 'bundler' (>= 0), here is why:

  Unable to download data from https://rubygems.org/ - no such name (https://rubygems.org/latest_specs.4.8.gz)

  The command "gem install bundler" failed and exited with 2 during setup.