[Gemfile] Pin rubocop to ~> 0.26.0

Rubocop 0.27.0 causes CocoaPods to have many offences
Release 0.35.0.rc1
Added note about RCs to changelog
Update CHANGELOG.md
[Resolver] Allow pre-release spec versions when a requirement has an external source

Closes https://github.com/CocoaPods/CocoaPods/issues/2768.