Update podspec to work with latest version of cocoapods (0.23.0)

The recent update to cocoapods means that the post-install code to compile the datamodel files is not longer needed (and in fact no longer works because of a change to the hook handling).

I've also submitted this to the master cocoapods specs repo but that's currently failing because of the Dropbox submodule dependency (UGH).

In the meantime, you can use the podspec by making a copy and referencing the copy in your Podfile:

    pod 'TICoreDataSync', :podspec => './TICoreDataSync.podspec'