Merge pull request #5295 from bwbarrett/v3.1.x-NEWS-pmix

dist: Update NEWS to include PMIx update

Author: bwbarrett

NEWS

@@ -57,6 +57,7 @@ included in the vX.Y.Z section and be denoted as:
 3.1.1 -- June, 2018
 -------------------
 
+- 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
   one-sided component should live.  Defaults to /dev/shm on Linux.