Add Travis badge

Author: pthariensflame

README.md

@@ -3,6 +3,7 @@
 [![Crates.io](https://img.shields.io/crates/v/refraction.svg)](https://crates.io/crates/refraction)
 [![Crates.io](https://img.shields.io/crates/l/refraction.svg)](https://raw.githubusercontent.com/pthariensflame/refraction/master/LICENSE)
 [![Crates.io](https://docs.rs/refraction/badge.svg)](https://docs.rs/refraction)
+[![Build Status](https://travis-ci.org/pthariensflame/refraction.svg?branch=master)](https://travis-ci.org/pthariensflame/refraction)
 
 A pure [Rust](https://www.rust-lang.org) [lens](https://www21.in.tum.de/teaching/fp/SS15/papers/17.pdf) library.
 This is a work in progress, and is not yet ready for production use.