Lock Rubocop version and make it happy

- Lock Rubocop version to 0.25.0
- Disable PerceivedComplexity check
- Fix minor issues in tests

Locking, as suggested by @meatballhat,  is useful to avoid difference between dev local settings and Travis CI one. As soon as this will get merged a rebase of #442 should solve the Travis complains for that PR.
I agree; hard-pinning increases the cost of upgrading later on, and I'd rather get fixes & incremental features as they come instead of forgetting about them, especially for a dev dependency.