Update DTCoreText pod

Updates the app to use the latest DTCoreText pod to resolve the crash on iOS 8.

Since WPiOS-Shared also has the DTCoreText dependency, it needed to be updated.  This PR changes the app's Podfile to point to the WordPress-iOS-Shared 0.0.6 tag on github instead of the cocoapod. This is a temporary change until the pod is updated. 

No need to merge this if we prefer to wait for the WordPress-iOS-Shared pod.

(Updating pods is scary :p)