Ignore link elements with data-premailer="ignore"

Should be able to ignore `link` elements, as we do with `style` elements.
Fix CSS selector
Add tests for ignore elements
Merge pull request #206 from johanbrook/patch-1

Add feature to ignore link elements with data attribute
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
htmlentities 4.3.2+ doesn't support ruby 1.8.x
bugfix regex infinite loop and resource eating
Merge pull request #217 from lipedjow/master

bugfix regex infinite loop and resource eating
Fix htmlentities version for ruby 1.8.7 support

htmlentities 4.3.2+ doesn't support ruby 1.8.x
Merge pull request #221 from karlhorky/patch-1

Fix htmlentities version for ruby 1.8.7 support
Add an option to enable or disable CSS @import statements.