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 do agree. I was unser about the behaviour of `~>` with a `0.x` version.

For example, see: http://guides.rubygems.org/patterns/#pessimistic_version_constraint. They say:
Using ~> with prerelease versions will restrict you to prerelease versions only. Is a `0.x` a pre-release? Anyway I'm OK with the proposed change :) and I'm going to update the PR.