Update README.md

README.md

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 # koa-varnish
 
+[![Build Status](https://travis-ci.org/vicanso/koa-varnish.svg?style=flat-square)](https://travis-ci.org/vicanso/koa-varnish)
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+
 The cache middleware for koa, it can cache the data such as varnish. The cache ttl is use the value of `s-maxage` or `maxage` from response header: `Cache-Control`.
 
 Uncacheable list: