Fix framework search paths to avoid compilation errors.

Commit 05c14cfdf6d9ebf9b7625b31b980bdca1aa4848a used $SDKROOT
to access those framework search paths, but $SDKROOT only expands
to the SDK name, while $PLATFORM_DIR expands to the full path of
the platform (this allows removing the hardcoding of the platform
names in the podfile).

Seems that using the $PLATFORM_DIR is the accepted answer from the community (see http://stackoverflow.com/questions/24275470/xctest-xctest-h-not-found-on-old-projects-built-in-xcode-6).

Note that I haven’t tested the Cocoapods changes, but they should work.