note/skip airplane-unfriendly test
rework gemfile/gemspec

 - lockdown rubydns version string
 - pull development dependencies into gemfile
 - move landrush into :plugins group for Vagrant
 - a wee wording change on the description
vagrant 1.5 support!

Vagrant 1.5 removed the concept of "global_config" from the
Vagrant::Environment, which was something that landrush used to get
access to its own plugin config.

Here we add support for the new way of properly accessing plugin config,
while maintaining support for older versions of vagrant.