Add remove_membership() method to Teams class (#807)

Per the Github v3 API [docs](https://developer.github.com/v3/teams/members/#deprecation-notice-2), the "remove team member" api is deprecated. This PR implements the new API call.

Deprecated API:
```
DELETE /teams/:team_id/members/:username
```

New API:
```
DELETE /teams/:team_id/memberships/:username
```

This PR also adds a `deprecation` notice to the docstring of the deprecated methods.

github/Team.py

@@ -149,6 +149,9 @@ class Team(github.GithubObject.CompletableGithubObject):
 
     def add_to_members(self, member):
         """
+        This API call is deprecated. Use `add_membership` instead.
+        https://developer.github.com/v3/teams/members/#deprecation-notice-1
+
         :calls: `PUT /teams/:id/members/:user <http://developer.github.com/v3/orgs/teams>`_
         :param member: :class:`github.NamedUser.NamedUser`
         :rtype: None
@@ -304,8 +307,23 @@ class Team(github.GithubObject.CompletableGithubObject):
         )
         return status == 204
 
+    def remove_membership(self, member):
+        """
+        :calls: `DELETE /teams/:team_id/memberships/:username <https://developer.github.com/v3/teams/members/#remove-team-membership>`
+        :param member:
+        :return:
+        """
+        assert isinstance(member, github.NamedUser.NamedUser), member
+        headers, data = self._requester.requestJsonAndCheck(
+            "DELETE",
+            self.url + "/memberships/" + member._identity
+        )
+
     def remove_from_members(self, member):
         """
+        This API call is deprecated. Use `remove_membership` instead:
+        https://developer.github.com/v3/teams/members/#deprecation-notice-2
+
         :calls: `DELETE /teams/:id/members/:user <http://developer.github.com/v3/orgs/teams>`_
         :param member: :class:`github.NamedUser.NamedUser`
         :rtype: None

github/tests/ReplayData/Team.testMembers.txt

@@ -107,3 +107,13 @@ None
 200
 [('status', '200 OK'), ('x-ratelimit-remaining', '4949'), ('content-length', '151'), ('server', 'nginx/1.0.13'), ('connection', 'keep-alive'), ('x-ratelimit-limit', '5000'), ('etag', '"8856425cedbdf3075576e823f39fc3d6"'), ('date', 'Sat, 26 May 2012 21:14:46 GMT'), ('content-type', 'application/json; charset=utf-8')]
 []
+
+https
+DELETE
+api.github.com
+None
+/teams/189850/memberships/jacquev6
+{'Authorization': 'Basic login_and_password_removed', 'User-Agent': 'PyGithub/Python'}
+None
+204
+[('status', '204 No Content'), ('x-ratelimit-remaining', '4968'), ('server', 'nginx/1.0.13'), ('connection', 'keep-alive'), ('x-ratelimit-limit', '5000'), ('etag', '"d41d8cd98f00b204e9800998ecf8427e"'), ('date', 'Sat, 26 May 2012 21:09:57 GMT')]

github/tests/Team.py

@@ -65,6 +65,7 @@ class Team(Framework.TestCase):
         self.assertFalse(self.team.has_in_members(user))
         self.team.add_membership(user, "maintainer")
         self.assertRaises(AssertionError, self.team.add_membership, user, "admin")
+        self.team.remove_membership(user)
 
     def testRepoPermission(self):
         repo = self.org.get_repo("FatherBeaver")