Document that newer than v1.1.x Vagrant is fine, too

The plugin works fine with at least Vagrant 1.2.x.

Also fix the RubyGems badge link.
Merge pull request #37 from tmatilai/readme

Document that newer than v1.1.x Vagrant is fine, too
fix CHANGELOG issue linking. fixes #29
Drop unneeded ConfigValidate action call

The built-in ConfigValidate action exits already earlier in the
middleware stack.
Merge branch 'pr/38'
update development/test dependancies
Ensure version is properly shell escaped. Fixes #27
update AWS acceptance test for latest `vagrant-aws`

The `ssh_username` has been removed and replaced by
`override.ssh.username`.
v1.1.1