- Move the ObjC tests into the list and exclude them on linux, this will change
  where in the order they start, since they are longer, it will have other
  things run in parallel instead of them ending up last and taking the longest.
- Switch to the Xcode 7.3 image.
- Drop the use of xctool and stream line things through the full_mac_build.sh
  script. This means we end up with only one build script instead of two.
- Tweaks to the mac build script:
  - Make iOS Xcode version support explicit
  - Support Debug/Release only building
  - Change the OS X min parallel count to 2 to better deal with VMs.
- Split the travis ios tests into the two Xcode Configurations as the logs are
  choking travis.