BLD: version strings should be updated only when tagging new release

0.13.0 was the first time I tagged a release and I got it wrong.
Version strings are bumped only when tagging a new release.

```
% grh v0.13.0rc1^               
% sudo python ./setup.py develop
% cat pandas/version.py
version = '0.12.0-1189-gd9b3340'
short_version = '0.12.0'
````
It makes sense since it leaves the next version undetermined until it happens.

Keeping the status-quo for now, until we agree on something better.