Fix Travis CI builds for Rails 2.3.16

~~This downgrades 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 excluded, but for now I left it in the matrix, because this PR just fixed it and should be able to prove that ;-)~~