Make the version number PEP 440 compliant

Building an egg from a development version of traits was causing bad version numbers (which are then corrected by newer egg infrastructure). Anyhow, it's a mess. There needs to be a number after the `.dev` part of the version. This adds that using code from enstaller.
@cournape: enstaller / PEP 440 question: will anything break if a `+` is used here instead of a `-`.  It's kinda annoying to have a PR for PEP 440 compliancy that doesn't create PEP 440 compliant version numbers.

Maybe we *should* just drop the local version label for now.  (Sorry.)