Add test helper

Since the changes for Rails 4 the TestCase duck-punches aren't as isolated.
Add this helper file so that everything gets loaded once.
No more calling 'load' instead of 'require'.
Uncomment the mailer tests. It was a mistake to comment them out.
Require TestCases before using them

Move the layout of minitest/rails around, help the tests pass.
Fix failing install generator test

The upgrades to the recent Rails security versions broke this. Dunno why.
Update travis config

Make travis work with Rails 4, which needs a prerelease of bundler to work.