Added travis-ci badge

Author: moul

README.md

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 Interact with Scaleway API from the command line.
 
+[![Build Status (Travis)](https://img.shields.io/travis/scaleway/scaleway-cli.svg)](https://travis-ci.org/scaleway/scaleway-cli)
 [![GoDoc](https://godoc.org/github.com/scaleway/scaleway-cli?status.svg)](https://godoc.org/github.com/scaleway/scaleway-cli)
 
 For node version, check out [scaleway-cli-node](https://github.com/moul/scaleway-cli-node).