Disable writing parsetab files to disk.

This fixes #206.

Before this change, when multiple Python processes are simultaneously
doing a `from flanker.addresslib import address`, it's possible for some
of them to crash in `ply` code.

See dabeaz/ply#184, where
I attempted to work around this issue by changing ply. You can see in this comment: dabeaz/ply#184 (comment) that the author of ply suggests two workarounds for this issue:

1. Remove `ply` as a dependency in setup.py and copy the source code of `ply` into `flanker`.
2. Disable writing parsetab files to disk when invoking `yacc`.

2) seemed like the simpler solution to me, so that's what I've done
here.