[Spec/Fixtures/SpecRepos/Master] Update
[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?`
[Documentation][Resolver] Remove references to #from_subspec_dependency? and talk instead about scoping by target platform
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.
[IntegrationSpec] Compare execution_output.txt
[Rakefile] Remove old references to compiling Xcodeproj

These days of compiled Xcodeproj gem are long gone.
[Rakefile] Remove references to Ruby 1.9

We don't support Ruby 1.9 anymore.
[Rakefile] Should be valid rubocop too
[Rakefile] Rubocop fixes
[IntegrationSpec] Mask the path of the git binary in execution_output.txt
[Gemfile] Move back to Cores master
Merge pull request #2780 from CocoaPods/scope-dependencies-by-consumer

[Resolver] Scope dependencies in #edge_is_valid_for_target? by the spec's consume for that target's platform
[Gemspec] Bump cocoapods-try to ~> 0.3.2
[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.
Ensure that HEAD pods are treated properly by using a new Specification::Set subclass