Add Travis badge

Author: Jim Porter

README.md

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 # bencode.hpp
 
+[![Travis build status][travis-image]][travis-link]
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 **bencode.hpp** is a small, header-only C++ library for parsing and generating
 [bencoded](https://en.wikipedia.org/wiki/Bencode) data.
 
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 ## License
 
 This library is licensed under the BSD 3-Clause license.
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+[travis-image]: https://travis-ci.org/jimporter/bencode.hpp.svg?branch=master
+[travis-link]: https://travis-ci.org/jimporter/bencode.hpp