Possible bug in isAvailableForServiceType

We ran into a problem where a user did not have whatsapp installed and this plugin did not recognize it properly. 
After investigation, we traced it down to a problem in [`SLComposeViewController isAvailableForServiceType`](https://developer.apple.com/library/prerelease/ios/documentation/NetworkingInternet/Reference/SLComposeViewController_Class/index.html#//apple_ref/occ/clm/SLComposeViewController/isAvailableForServiceType:). Passing the string "whatsapp" oddly returned true for this method; actually any string not in the [defined constants](https://developer.apple.com/library/prerelease/ios/documentation/Social/Reference/SLRequest_Class/index.html#//apple_ref/doc/constant_group/Service_Type_Constants) returned true in our tests.

The proposed fix first checks if the type is in the defined constants and return false if not.