Add travis build badge to README

Author: mxstbr

README.md

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 Public analytics as a Node.js microservice, no sysadmin experience required.
 
+[![Build Status](https://travis-ci.org/mxstbr/micro-analytics.svg?branch=master)](https://travis-ci.org/mxstbr/micro-analytics)
+
 A tiny analytics server with less than 100 lines of code, easy to run and hack around on. It does one thing, and it does it well: count the views of something and making the views publicly accessible via an API.
 
 (there is currently no frontend to display pretty graphs, feel free to build one yourself!)