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.
Please could you add a comment above this line with a brief explanation (e.g., `# autogenerated files`).