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.
I had it as `+`, but when I created an sdist, the egg name had a `-`. I wasn't sure what to make of that.