Revise for 3.1.1 release. Taken from
https://github.com/openbabel/openbabel/commit/650b1f829ac47cc84877a330366ec9485da3ec99

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -24,7 +24,7 @@ include (CheckCXXCompilerFlag)
 # had too many 1.100.1 releases. :-)
 set(BABEL_MAJ_VER   3)
 set(BABEL_MIN_VER   1)
-set(BABEL_PATCH_VER 0)
+set(BABEL_PATCH_VER 1)
 
 # This should be phased out in preference for just using the target name
 set(BABEL_LIBRARY openbabel)