Add a coverage badge

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-[![Build Status](https://travis-ci.org/danvk/dygraphs.svg?branch=markdown-readme)](https://travis-ci.org/danvk/dygraphs)
+[![Build Status](https://travis-ci.org/danvk/dygraphs.svg?branch=markdown-readme)](https://travis-ci.org/danvk/dygraphs) [![Coverage Status](https://img.shields.io/coveralls/danvk/dygraphs.svg)](https://coveralls.io/r/danvk/dygraphs)
 # dygraphs JavaScript charting library
 
 The dygraphs JavaScript library produces interactive, zoomable charts of time series: