Hi James,
It looks like you might have a semi-broken installation. In your output I see "
/usr/local/bin/vol.py" and also "/opt/volatility-2.2". When you installed, did you run setup.py or did you just extract the archive you downloaded from https://code.google.com/p/volatility/downloads/list and copy vol.py to /usr/local/bin? Is this the only version of volatility you installed or have you used older versions in the past that might not have been removed properly?
I'm positive the error doesn't occur in "normal" installations. You might try running "make clean" inside the /opt/volatility-2.2 directory to remove any stale/old compiled .pyc files and then re-running yarascan.
Let us know how it goes,
MHL