add coverage status

.travis.yml

@@ -9,7 +9,6 @@ addons:
 
 # run builds for both all the trains
 rust:
-  - nightly
   - beta
   - stable
 

README.md

@@ -3,6 +3,7 @@
 [![crates.io](http://meritbadge.herokuapp.com/coap)](https://crates.io/crates/coap)
 [![Travis Build Status](https://travis-ci.org/Covertness/coap-rs.svg?branch=master)](https://travis-ci.org/Covertness/coap-rs)
 [![Windows Build Status](https://ci.appveyor.com/api/projects/status/ic36jdu4xy6doc59?svg=true)](https://ci.appveyor.com/project/Covertness/coap-rs)
+[![Coverage Status](https://coveralls.io/repos/Covertness/coap-rs/badge.svg?branch=master&service=github)](https://coveralls.io/github/Covertness/coap-rs?branch=master)
 [![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](./LICENSE)
 
 A [Constrained Application Protocol(CoAP)](https://tools.ietf.org/html/rfc7252) library implemented in Rust.