Mark User.gravatar() as deprecated

We're using self-hosted avatars instead.
See `pillar.api.users.avatar.url()` in the Pillar source code instead.

pillarsdk/users.py

@@ -14,7 +14,12 @@ class User(List, Find, Create, Post, Update, Delete):
     path = "users"
 
     def gravatar(self, size=64):
-        """Generate Gravatar link on the fly using the email value"""
+        """Deprecated: return the Gravatar URL.
+
+        .. deprecated::
+            Use of Gravatar is deprecated, in favour of our self-hosted avatars.
+            See pillar.api.users.avatar.url(user).
+        """
         parameters = {'s':str(size), 'd':'mm'}
         return "https://www.gravatar.com/avatar/" + \
             hashlib.md5(self.email.lower()).hexdigest() + \