docs: Add Travis badge to README.md

README.md

 # Feather
 
+[![Travis branch](https://img.shields.io/travis/colebemis/feather/master.svg?style=flat-square)](https://travis-ci.org/colebemis/feather)
+
 ## What is Feather?
 
 Feather is a collection of **simply beautiful open source icons**. Each icon is designed on a 24x24 grid with an emphasis on simplicity, consistency and legibility.