Changing Version and Bundle keys to better fit Xcode paradigm

Changed CFBundleVersion to refer to $ios_app_build and added CFBundleShortVersionString to refer to $ios_app_version, as discussed in this Stack Overflow answer:

http://stackoverflow.com/questions/6851660/version-vs-build-in-xcode-4

Note: this pull request requires another special mixpanel attribute: $ios_app_build to refer to the build number.