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.
First thought here: why bother catching the `ImportError` at all?  None of the rest of Traits will work if e.g., `ctraits` hasn't been built.

Second thought: this is going to cause problems with the Read the Docs doc build.  That's currently broken anyway, but this is going to make it harder: we can't do a `python setup.py build` for the RtD docs, but we *do* want a decent version number.