Update Rails version

- Remove specific rails version from Gemfile, and set upper limit
- Relax mocha version
```
Solve this:
[  20/2209] AuthorizeNetCimTest#test_should_create_customer_payment_profile_request/Users/sane/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/mocha-0.11.4/lib/mocha/integration/mini_test/version_230_to_262.rb:36:in `rescue in run': uninitialized constant Mocha::Integration (NameError)
```