Merge tag 'v0.31.1'

Changes in synapse v0.31.1 (2018-06-08)
=======================================

v0.31.1 fixes a security bug in the ``get_missing_events`` federation API
where event visibility rules were not applied correctly.

We are not aware of it being actively exploited but please upgrade asap.

Bug Fixes:

* Fix event filtering in get_missing_events handler (PR #3371)