[XCConfigIntegrator] Assign xcconfigs to all targets being integrated.

Fixes #2752.
[Travis] Make autoload load `Xcodeproj::PlistHelper`.
[CHANGELOG] Tune grammar and fix spacing issues.
[Gemspec] Update xcodeproj dependency to `~> 0.20.1`
[Gemspec] Update cocoapods-try dependency to `~> 0.4.2`
[bundle] Explicitely test against json version that comes with OS X.
[CHANGELOG] Highlight call to test new resolver.
[Gemfile] Add cocoapods-dependencies
[Validator] Add private mode
[Repo::Push] Default to --private