Increment version to VTK 8.1.2

Saving off current release before making 8.2.0 branch

CMake/vtkVersion.cmake

 # VTK version number components.
 set(VTK_MAJOR_VERSION 8)
 set(VTK_MINOR_VERSION 1)
-set(VTK_BUILD_VERSION 1)
+set(VTK_BUILD_VERSION 2)