chore(README): add travis build badge
# gxi
-[![Build Status](https://drone.exqa.de/api/badges/Cogitri/gxi/status.svg)](https://drone.exqa.de/Cogitri/gxi)
+[![Drone CI Build Status](https://drone.exqa.de/api/badges/Cogitri/gxi/status.svg)](https://drone.exqa.de/Cogitri/gxi)
+[![Travis CI Build Status](https://travis-ci.com/Cogitri/gxi.svg?branch=master)](https://travis-ci.com/Cogitri/gxi)
GTK frontend, written in Rust, for the [xi editor](https://github.com/google/xi-editor).
