Add coveralls badge

README.md

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-RxLua [![Build Status](https://travis-ci.org/bjornbytes/RxLua.svg)](https://travis-ci.org/bjornbytes/RxLua)
+RxLua [![Build Status](https://travis-ci.org/bjornbytes/RxLua.svg)](https://travis-ci.org/bjornbytes/RxLua) [![Coverage Status](https://coveralls.io/repos/github/bjornbytes/RxLua/badge.svg?branch=master)](https://coveralls.io/github/bjornbytes/RxLua?branch=master)
 ===
 
 [Reactive Extensions](http://reactivex.io) for Lua.