Fix build status in README.md

README.md

 og-rek
 ======
 [![GoDoc](https://godoc.org/github.com/kisielk/og-rek?status.svg)](https://godoc.org/github.com/kisielk/og-rek)
-[![Build Status](https://travis-ci.org/kisielk/og-rek?branch=master)](https://travis-ci.org/kisielk/og-rek)
+[![Build Status](https://travis-ci.org/kisielk/og-rek.svg?branch=master)](https://travis-ci.org/kisielk/og-rek)
 ogórek is a Go library for encoding and decoding pickles.