Update .travis.yml to use available rubies and add 2.2.0
Update .travis.yml to use available rubies and add 2.2.0

rhrfonseca/droplet_kit,molisoft/droplet_kit,ledestin/droplet_kit,wicky-info/droplet_kit,zachaysan/droplet_kit,alambike/droplet_kit,andrewsomething/droplet_kit,digitalocean/droplet_kit