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.