git: 3.15.5 added to rhel6 variants

Programming/cmake/files/variants.rhel6

@@ -0,0 +1,11 @@
+cmake/2.8.12.2       stable
+cmake/3.1.3          stable
+cmake/3.3.2          stable
+cmake/3.4.1          stable
+cmake/3.6.3          stable
+cmake/3.8.1          removed
+cmake/3.9.6          stable
+cmake/3.10.3         stable b:gcc/7.3.0
+cmake/3.13.3         stable b:gcc/7.3.0
+cmake/3.14.0         stable b:gcc/7.3.0
+cmake/3.15.5         stable b:gcc/7.3.0 b:patchelf/0.8.1