Updates pod spec

- Removes PivotalCoreKit/UIKit/SpecHelper/Base as it only included a
  header that would try to include other files not available.
- Adds the convenience header to the
  PivotalCoreKit/UIKit/SpecHelper/Extensions subpod
- Removes PivotalCoreKit/Foundation/SpecHelper/Base again including only
  header and no other files.
- Adds dependency on Fakes to Fixtures.
- Pod no longer depends on Cedar in any way.
- Returns source to main CoreKit repo (instead of fork)