Add support for Python3.6 lib search with Cmake (#527)

* Add support for Python3.6 lib search with Cmake

A recent commit added support for python3.5 lib search with Cmake. This PR extends that to python 3.6.

Relevant old commit: https://github.com/davisking/dlib/commit/17a2c75ddffb13e915686803c5e3bf4cc1389a96

* Turn python versions into a list

dlib/cmake_utils/add_python_module

@@ -68,7 +68,7 @@ if (PYTHON3)
     if (NOT Boost_FOUND)
         FIND_PACKAGE(Boost 1.41.0 COMPONENTS python)
     endif()
-    set(Python_ADDITIONAL_VERSIONS 3.5)
+    set(Python_ADDITIONAL_VERSIONS 3.5 3.6)
     FIND_PACKAGE(PythonLibs 3.4 REQUIRED)
 else()
     FIND_PACKAGE(Boost 1.41.0 COMPONENTS python)