Better dimensions and border-radius

This should match the badges to shields.io ones.

Before:
![before](https://cloud.githubusercontent.com/assets/115237/5127965/10d5604e-70d8-11e4-96c6-474a9f3f3971.png)
After:
![after](https://cloud.githubusercontent.com/assets/115237/5127966/10fbd940-70d8-11e4-847e-1cf58f7e0dd1.png)