dist: Update NEWS for 3.1.1rc2

Signed-off-by: Brian Barrett <[email protected]>

Author: bwbarrett

NEWS

@@ -57,6 +57,7 @@ included in the vX.Y.Z section and be denoted as:
 3.1.1 -- June, 2018
 -------------------
 
+- Fix potential hang in UCX PML during MPI_FINALIZE
 - Update internal PMIx to v2.1.2rc2 to fix forward version compatibility.
 - Add new MCA parameter osc_sm_backing_store to allow users to specify
   where in the filesystem the backing file for the shared memory