Depend on riak>=2.1 because we need client.close().
Support paginated index queries and optionally return terms.
Merge branch 'develop' into feature/issue-850-riak-paginated-indexes

Conflicts:
	vumi/persist/riak_manager.py
	vumi/persist/txriak_manager.py
Update some manager method docstrings.
Ensure that the continuation value is unicode and assert this in tests.
Better docstrings.
Merge branch 'feature/issue-850-riak-paginated-indexes' into develop
Merge branch 'develop' of github.com:praekelt/vumi into develop