Speed up nokogiri installation during Travis runs

Version 1.6 of Nokogiri builds its own copy of libxml2 during installation.
Setting `NOKOGIRI_USE_SYSTEM_LIBRARIES=true` skips this step and shaves about a
minute off of Travis runs.

Inspired by https://github.com/mitchellh/vagrant/commit/dd575b97089f626b899d02502abb1cc8c01a8aed