[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
[Bundle] Update Core
Merge pull request #2893 from CocoaPods/seg-podspec-eval-context

[ExternalSource] Store downloaded external source podspecs as JSON, evaluated from the context of the source directory