Reduce size of output file after upgrading to criterium 0.4.2

Criterium 0.4.2 returns the full benchmark expression return values in
the map it returns.  The previous version I was using only had the
.hashCode values of the expression return values, which was much
smaller to print in most cases.  Do not change criterium, but replace
the :results key with a :result-hashes key that contains only the
result hashes.
Update results2graphs for handling cljexprs output files