Update "friendship_exists" api query to v1.1

v1.1 of Twitter API added authentication as a requirement and changed the endpoint for looking the relationship between two followers - see docs here https://dev.twitter.com/docs/api/1.1/get/friendships/lookup

I changed the endpoint URL from "exists" to "lookup" to reflect the correct endpoint, but left the function name "friendship_exists" as is in order not to break existing code.