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 need to be a number after the `.dev` part of the version. This adds that using code from enstaller.
Actually, why is the `decode('ascii')` there? IIUC, this value then gets inserted into a (plain `str`) template via a format comment, so it shouldn't be `unicode` in the first place.