[Resolver] Scope dependencies in #edge_is_valid_for_target? by the spec's consume for that target's platform

We don't support Ruby 1.9 anymore.

Closes https://github.com/CocoaPods/CocoaPods/issues/2759.

[Command] Don't check for git >= 1.7.5

Update sources option documention for multiple sources.

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

Thanks to @pietbrauer

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

Reverts 4ab76e80

[Documentation][Resolver] Remove references to #from_subspec_dependency? and talk instead about scoping by target platform

Closes https://github.com/CocoaPods/CocoaPods/issues/2779.

[Resolver] Scope dependencies in #edge_is_valid_for_target? by the spec's consume for that target's platform

This replaces the hack of `Dependency#from_subspec_dependency?`

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

Rubocop 0.27.0 causes CocoaPods to have many offences

See #2757.

[WIP] [Resolver] Add filtering for subspecs included via subspec_dependencies

[ResolverSpec] Add spec covering the exclusion of subspec dependencies of incompatible platforms when there's only one target platform

Done by using AFNetworking 2.4.1 and its UIKit subspec (iOS only) on an OS X platform

Fixes #2752.