add travis badge

Author: developit

README.md

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 # preact-cli-plugin-async
 
-[![npm](https://img.shields.io/npm/v/preact-cli-plugin-async.svg)](https://npm.im/preact-cli-plugin-async) [![Greenkeeper badge](https://badges.greenkeeper.io/developit/preact-cli-plugin-async.svg)](https://greenkeeper.io/)
+[![npm](https://img.shields.io/npm/v/preact-cli-plugin-async.svg)](https://npm.im/preact-cli-plugin-async) [![travis](https://travis-ci.org/developit/preact-cli-plugin-async.svg?branch=master)](https://travis-ci.org/developit/preact-cli-plugin-async) [![greenkeeper](https://badges.greenkeeper.io/developit/preact-cli-plugin-async.svg)](https://greenkeeper.io/)
 
 [Preact CLI] plugin that adds optimized support for async/await via [Kneden].