Added travis badge

Will enable people check build status from github project-page.

README.markdown

@@ -1,6 +1,8 @@
 Redigo
 ======
 
+[![Build Status](https://travis-ci.org/garyburd/redigo.svg?branch=master)](https://travis-ci.org/garyburd/redigo)
+
 Redigo is a [Go](http://golang.org/) client for the [Redis](http://redis.io/) database.
 
 Features