bump version and changelog

CHANGES.rst

-Changes in synapse v0.31.0-rc1 (2018-06-04)
-==========================================
+Changes in synapse v0.31.0 (2018-06-06)
+=======================================
 
 Most notable change is to switch to python prometheus library to improve system
 stats reporting. WARNING this changes a number of prometheus metrics in a
 backwards-incompatible manner. For more details, see
 `docs/metrics-howto.rst <docs/metrics-howto.rst#removal-of-deprecated-metrics--time-based-counters-becoming-histograms-in-0310>`_.
 
+Bug Fixes:
+
+ * Fix metric documentation tables (PR #3341)
+ * Fix LaterGuage error handling (694968fa81aab4eac81309b1e16f6063103dd57f)
+ * Fix replication metrics (b7e7fd2d0edd4d46aaf9d6afc8df14cf3de911f9)
+ 
+Changes in synapse v0.31.0-rc1 (2018-06-04)
+==========================================
+
 Features:
 
 * Switch to the Python Prometheus library (PR #3256, #3274)

synapse/__init__.py

@@ -16,4 +16,4 @@
 """ This is a reference implementation of a Matrix home server.
 """
 
-__version__ = "0.31.0-rc1"
+__version__ = "0.31.0"