Fix rails 2.3.16 gemfile for ruby 2.0.0

This allows the travis specs to pass on rails 2.3.16 / ruby 2.0.0.
Update build matrix excludes for ruby 2.0.0
Re-exclude exotic rails 2.3.16 / ruby 2.0.0 build from travis ci

We know the build works fine, no need to test such an exotic combination
all the time.
Merge pull request #387 from felixbuenemann/fix-travis-ci-builds

Fix Travis CI builds for Rails 2.3.16