fix nokogiri 1.6 incompatibility issue, thanks @benjaminwood

As reported by @benjaminwood in https://github.com/locomotivecms/engine/issues/721 the last sanitize version is not compatible with other gems of the engine anymore.

This is a very simple pull request to force sanitize version to 2.0.3, Waiting for other gem with nokogiri as a dependency to update to >=2.0.4