fix doc refs to https

Author: bckohan

doc/source/refs.rst

@@ -1,5 +1,5 @@
 
-.. _GitHub: http://github.com/bckohan/enum-properties
-.. _PyPI: http://pypi.python.org/pypi/enum-properties
+.. _GitHub: https://github.com/bckohan/enum-properties
+.. _PyPI: https://pypi.python.org/pypi/enum-properties
 .. _Enum: https://docs.python.org/3/library/enum.html#enum.Enum
 .. _ValueError: https://docs.python.org/3/library/exceptions.html#ValueError