[IntegrationSpec] Fixed after Reachability update
[Validator] Fix linting subspecs that have a higher deployment target than the root spec

Closes https://github.com/CocoaPods/CocoaPods/issues/1919.
[Bundle] Update Molinillo
[IntegrationSpec] Use fixture spec repo
[Gemfile] Update CLIntegracon
[IntegrationSpec] Don't leak to stdout

The path of the mercury binary leaked to the spec runner output
[IntegrationSpec] Update for changed ENV variables
Merge pull request #2899 from CocoaPods/fix_integration_spec_after_reachability_update

[IntegrationSpec] Fixed after Reachability update
[ExternalSource] Store downloaded external source podspecs as JSON, evaluated from the context of the source directory
[IPC] Use new #to_pretty_json method
[Gemfile] Use Core from master
[Changelog] Add entry for external source podspecs being evaluated in the source directory and stored as JSON