Remove dashes from license field

Hey, this contains a fix for a very small issue encountered when a license
text contains a dash. E.g. if the license is "non-free". At the moment,
this will generate the following shield:

![shield](http://img.shields.io/badge/license-non-free-blue.png)

After the patch:

![shield](http://img.shields.io/badge/license-non_free-blue.png)

The same issue probably exists for other fields as well.