[LockingDependencyAnalyzer] Ensure vertex exists before detaching it

Closes https://github.com/CocoaPods/CocoaPods/issues/2779.
[Bundle] update.
Update sources option documention for multiple sources.
[Command] Don't check for git >= 1.7.5

Git 1.7.5 is no longer depended on due to
https://github.com/CocoaPods/Core/commit/99748b5b9fa8fe6541218f1752c0379589c7a6ee

Reverts 4ab76e807cb7bae409ee058d63dd6bfd3dac77ca
[Resolver] Compare against locked dependency upfront

This adds a forward checking step that proves to be a major optimization when there are unresolvable conflicts involving the Lockfile

Thanks to @pietbrauer
Merge pull request #2784 from postmechanical/master

Update sources option documention for multiple sources.
Merge pull request #2783 from CocoaPods/remove-git-1.7.5

[Command] Don't check for git >= 1.7.5
[PodSourceInstaller] Dont attempt to clean installation if root does not exist

Closes https://github.com/CocoaPods/CocoaPods/issues/2759.
[Rakefile] Remove references to Ruby 1.9

We don't support Ruby 1.9 anymore.