Add required Ruby version to gemspec
Add required Ruby version to gemspec

With the addition of some keyword arguments, droplet_kit will now require at least ruby 2.0.0.
rhrfonseca/droplet_kit,ledestin/droplet_kit,alambike/droplet_kit,andrewsomething/droplet_kit,zachaysan/droplet_kit,digitalocean/droplet_kit,wicky-info/droplet_kit,molisoft/droplet_kit