PE-1504: drop ruby 1.8.7 and add ruby 2.0.0 to the projects's travis-ci matrix

This patch updates the project's travis-ci matrix as stated in the title to make the tests more useful. (As the tests didn't run with ruby 1.8.7 because the prerequisite ```nokogiri``` gem requires ruby version >= 1.9.2 and ruby 2.0.0 was not covered at all.)