Closes #2759

Fix :head pods

[Rakefile] Remove old references to compiling Xcodeproj

[IntegrationSpec] Compare execution_output.txt

[Resolver] Fix crash when there are lockfile dependencies that were never actually installed (i.e. incompatible platform subspecs)

See https://github.com/CocoaPods/CocoaPods/issues/2789#issuecomment-61797151.

[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.

These days of compiled Xcodeproj gem are long gone.

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.