[Rubocop] trying to split CocoaPods/Core specific rubocop rules and CP-common rules, so that we start using a common .rubocop-cocoapods.yml across all CP repos
See discussion in CocoaPods/cocoapods-plugins#17
[Rubocop] Adding exclusion of `spec/fixtures/**` in the `.rubocop-cocoapods.yml` common rules config file
even if it has to be repeated in the .rubocop.yml config file if we need to add other exclusions (like `lib/*/vendor/**`)
Merge pull request #126 from CocoaPods/rubocop-common

[Rubocop] Use a .rubocop-cocoapods.yml config file common to all CP repos