[Specification::DSL] Add prepare_command

Closes #1274
Merge branch 'master' into feature-prepare_command

* master:
  [Gemfile] Require cocoapods-core from the master branch
  [Gemspec] Bump dependencies version
[Gemfile] Require cocoapods-core from the feature-prepare_command branch
Merge pull request #1252 from CocoaPods/feature-prepare_command

[Specification::DSL] Add prepare_command