Update license badge.
:target: https://pypi.python.org/pypi/law
:alt: Package version
-.. image:: https://img.shields.io/pypi/l/law.svg?style=flat
- :target: https://pypi.python.org/pypi/law
+.. image:: https://img.shields.io/github/license/riga/law.svg
+ :target: https://github.com/riga/law/blob/master/LICENSE
:alt: License
