BLD: consider version tags only when using git-describe to gen version string
On master we use tags strictly to indicate releases.
In other clones, this may not be the case. Specifically, jenkins
sometimes likes to tags each build which can wreak [havoc](http://pandas.pydata.org/pandas-build/dev/) when
using git-describe naively to generate version strings.

git allows you to filter the tags to consider when constructing the version string. well then.

I tested this locally of course, but as always with changes to setup.py - duck and cover.
