Use travis for building

- add a .travis.yml file
- add the badge to the README.md

.travis.yml

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+language: go
+
+go:
+- 1.12.x

README.md

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+[![Build Status](https://travis-ci.com/elisescu/tty-share.svg?branch=master)](https://travis-ci.com/elisescu/tty-share)
+
 # tty-share
 
 It is a very simple command line tool that gives remote access to a UNIX terminal session. It's using the [PTY](https://en.wikipedia.org/wiki/Pseudoterminal) system, so it should work on any *UNIX* system (Linux, OSX). Because it's written in GO, the tool will be a single binary, with no dependencies, which will also work on your ARM Raspberry Pi.