Fix Travis CI builds for Rails 2.3.16

This downgraded rubygems to 1.8.25 for compatibility with rails 2.3.16. It also updates the rails 2.3.16 gemfile so it works with MRI ruby 2.0, which no longer loads iconv by default.

I've also fixed some bundler deprecation warning and updated the build matrix exludes and no longer ignore ruby 2.0.0 build failures.

In the future the ruby 2.0.0 / rails 2.3.16 combination should also be exluded, but for now I left it in the matrix, because this PR just fixed it and should be able to prove that ;-)