Test against Rails HEAD (4, beta)

Rails 4 is only supported on Ruby 1.9 so the combination of Ruby 1.8 and Rails 4 is excluded from the Travis build matrix.
Update Bundler before install Rails 4
Update Bundler before *install* rather than before the build
Fix typo in Travis build matrix exclusion
Cleaning up the bundler gem is not necessary (nor possible, it seems)
Apparently, Rails 4 requires Ruby 1.9.3+