Bump version since 0.4 has been released some time ago.

Since version 0.4 is released and available on [pypi](https://pypi.python.org/pypi/numpydoc/0.4) 
```pip list --outdated``` will report an install from the master branch (0.4.dev) as outdated since 0.4.dev < 0.4. 
This just bumps the version to fix that it seems in line with standard pre release version numbers and the ones used for numpy.