Create separate Gemfile for Ruby 1.8.7

* Allows users of Ruby 1.9 (and above) to use the latest version of `nokogiri`
* Allows users of Ruby 1.8.7 to continue using `nokogiri` version 1.5
* Creates a separate `Gemfile` for Ruby 1.8.7, where this project can specify stricter version constraint on other dependencies that may drop Ruby 1.8.7 support in future minor releases
* Modifies the CI configuration to run tests against this new Gemfile on Ruby 1.8.7