Fix the build

Without a version specifier Travis will attempt to install activesupport 4.0.0, which will fail with `Gem::InstallError: activesupport requires Ruby version >= 1.9.3`

For example: https://travis-ci.org/stripe/stripe-ruby/jobs/8611599

Hopefully this fixes it. Tested locally with the default-with-activesupport.gemfile on 1.8.7, 1.9.3, and 2.0.0.