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 know this is a very minor nitpick, but can the version spec be `'~> 0.25'`?