enable github actions support for PRs

Furthermore we disable travis support and link the right badge in the
README. This is supposed to fix #55

Signed-off-by: Christian Rebischke <chris@nullday.de>

.github/workflows/main.yml

-name: CI
+name: Github-Actions
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
-  build:
+  vagrant-validation:
 
     runs-on: ubuntu-latest
-    
+
     steps:
     - uses: actions/checkout@v1
     - name: install packer

.travis.yml

-language: generic
-dist: trusty
-sudo: false
-
-install:
-- ./generic-ci.sh install
-
-script:
-- ./generic-ci.sh verify
-
-notifications:
-- email: false

README.md

 # arch-boxes
-[![Build Status](https://travis-ci.org/archlinux/arch-boxes.svg?branch=master)](https://travis-ci.org/archlinux/arch-boxes)
+[![Actions Status](https://github.com/archlinux/arch-boxes/workflows/CI/badge.svg)](https://github.com/archlinux/arch-boxes/actions)
 
 Arch-boxes provides automated builds of the Arch Linux releases for
 different providers and post-processors. Check the providers or post-processor sections if you want to know