Conditionally include `sudo` in suggested update command

Closes https://github.com/CocoaPods/CocoaPods/issues/2058
Merge branch 'check-if-sudo-needed-for-gem-install' of https://github.com/segiddins/CocoaPods into segiddins-check-if-sudo-needed-for-gem-install

* 'check-if-sudo-needed-for-gem-install' of https://github.com/segiddins/CocoaPods:
  Conditionally include `sudo` in suggested update command

Conflicts:
	spec/unit/sources_manager_spec.rb
Merge branch 'segiddins-check-if-sudo-needed-for-gem-install'

* segiddins-check-if-sudo-needed-for-gem-install:
  Conditionally include `sudo` in suggested update command