Merge branch '62-boost-release-1-70-0-with-gcc-7-3-0-openmpi-3-1-3-as-stable' into 'master'

Resolve "boost: release 1.70.0 with gcc/7.3.0, openmpi/3.1.3 as stable"

Closes #62

See merge request Pmodules/buildblocks!76

MPI/boost/files/variants.rhel6

@@ -6,7 +6,6 @@ boost/1.68.0         stable	gcc/7.3.0 openmpi/3.0.0 b:Python/2.7.14
 boost/1.68.0         stable	gcc/7.3.0 openmpi/3.1.2 b:Python/2.7.14
 boost/1.68.0         stable	gcc/7.3.0 openmpi/3.1.3 b:Python/2.7.14
 boost/1.68.0-1       stable	gcc/7.3.0 openmpi/3.1.3 b:zlib/1.2.11
-boost/1.70.0         unstable	gcc/7.3.0 openmpi/3.1.3 b:zlib/1.2.11
 
 boost/1.68.0         stable	gcc/8.2.0 openmpi/1.10.7 b:Python/2.7.14
 boost/1.68.0         stable	gcc/8.2.0 openmpi/2.1.5 b:Python/2.7.14
@@ -15,3 +14,5 @@ boost/1.68.0         stable	gcc/8.2.0 openmpi/3.1.2 b:Python/2.7.14
 boost/1.68.0         stable	gcc/8.2.0 openmpi/3.1.3 b:Python/2.7.14
 
 boost/1.68.0-1       stable	gcc/7.3.0 mpich/3.3 b:zlib/1.2.11
+
+boost/1.70.0         stable	gcc/7.3.0 openmpi/3.1.3 b:zlib/1.2.11