Added Travis build status icon

Author: sebright

README.md

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 json-rpc-server
 ===============
+[![Build Status](https://travis-ci.org/grayjay/json-rpc-server.svg?branch=master)](https://travis-ci.org/grayjay/json-rpc-server)
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 An implementation of the server side of JSON RPC 2.0. See <http://www.jsonrpc.org/specification>. This library uses ByteString for input and output, leaving the choice of transport up to the user. The documentation is on Hackage: <http://hackage.haskell.org/package/json-rpc-server>.