[Resolver] Add filtering for subspecs included via subspec_dependencies
[Resolver] Fix scoping of subspec dependencies by maintaining from_subspec_dependency? attribute
[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
[Resolver] Rename matching_dependencies_from_node lamdba to be more readable
[ResolverSpec] Clarify spec description
[Resolver] Extract valid_dependencies_for_target_from_node method
[Resolver] Extract edge_is_valid_for_target? method
Merge pull request #2760 from CocoaPods/seg-from-subspec-dependencies

[WIP] [Resolver] Add filtering for subspecs included via subspec_dependencies
[Gemfile] Use Core from master
[bundle] update
[Resolver] More tests for activating subspecs for requested platform.

See #2757.
[Gemfile] Pin rubocop to ~> 0.26.0

Rubocop 0.27.0 causes CocoaPods to have many offences
Release 0.35.0.rc1
Added note about RCs to changelog
Update CHANGELOG.md
[Resolver] Allow pre-release spec versions when a requirement has an external source

Closes https://github.com/CocoaPods/CocoaPods/issues/2768.
[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?`
[LockingDependencyAnalyzer] Ensure vertex exists before detaching it

Closes https://github.com/CocoaPods/CocoaPods/issues/2779.
[Documentation][Resolver] Remove references to #from_subspec_dependency? and talk instead about scoping by target platform