Building on a machine still using Python 2.7 as its default

I found a problem running Make on Linux Mint 18.1 because this defaults to calling Python 2.7.12 and PyYAML requires Python 3. It can be fixed by explicitly calling python3 in make_dungeon.py

It may be worth updating INSTALL.adoc to include a suggestion to try editing make_dungeon.py to change #!/usr/bin/env python to read #!/usr/bin/env python3 in the event Make returns the following message:

./make_dungeon.py Traceback (most recent call last): File "./make_dungeon.py", line 10, in import sys, yaml ImportError: No module named yaml