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

As discussed in CocoaPods/cocoapods-plugins#17 ([specifically here](https://github.com/CocoaPods/cocoapods-plugins/pull/17#discussion_r12218753))

Trying to split the rubocop rules to have rules specific to CocoaPods/Core separated to rules that should be common to all CP repos, like CocoaPods/CocoaPods but also CP plugins.
This way we could have every CP repo `inherit_from` the same `.rubocop-cocoapods.yml` and then only throw their specific rules

/c @dbgrandi @irrationalfab