[Gemfile] Remove libxml2

This has no longer been a dependency of Xcodeproj

Drop support for Ruby 1.8.7

[ResolverSpec] Add a test case for subspec dependencies where not all versions have a subspec with the given name

Fixes #2733.

Molinillo

See https://github.com/CocoaPods/CocoaPods/pull/2637

[Analyzer][LockingDependencyAnalyzer] Use new LockingDependencyAnalyzer to generate_version_locking_dependencies

Tis should now behave more like Bundler

[ResolverSpec] Add uses a Podfile requirement even when a previously declared dependency has a different requirement

This was tough to do, since it really adversely affected performance on unresolvable podfiles. Hence the optimization of making sure that a pod's dependencies with the same root name are pinned to the same version in `dependencies_for`

\c @kylef, @alloy

Addresses #3 of https://github.com/CocoaPods/CocoaPods/pull/2637#issuecomment-58584402