Add Rails 4.0 fixture app
Regenerate db/schema.rb for rails 4.0 fixture
Add gem source to make Bundler find remote gems
Run bundle update commands in a clean environment

Without this, bundle update will not attempt to fetch remote gems.
Update minitest dependency
DRY up integration tests

Adding a new Rails version fixture should now be a matter of adding one
line to the VERSIONS array.
Drop support for Rails 2.3

Rails 2.3 is no longer supported by the Rails team. In addition, it does
not run on Ruby 2.0 and up. To use memory_test_fix with Rails 2.3, use
released version 0.2.2.
Include Rails 4.0 in the integration tests