BUG: Fix update when new egg matches one in lower prefix.

In case of multiple prefixes, an update would not be performed
in primary prefix if a lower prefix has the updated version,
despite the primary prefix having an older version of the egg
installed.
Merge pull request #138 from pankajp/fix-chained-overridden-update

BUG: Fix update when new egg matches one in lower prefix.

LGTM, merging.