Fix #195. RPM package provider broken on AIX.

Prior to this commit, the RPM package provider would barf when
attempting to run `self.instances` on AIX. The culprit is the archaic
version of RPM (3.0.5) on AIX. The RPM provider assumed that the
`--nodigest` option (introduced in 4.0.2, circa 2001) was available.
This commit removes the `--nodigest` option for RPM versions older than
4.0.2.