Perform validation at action execution time. Fixes #13

By default Vagrant performs config validation for ALL plugins at EVERY
command execution. This would cause simple commands like `vagrant ssh` 
or `vagrant status` to fail if:

* `omnibus.chef_version` was not configured in a `Vagrantfile`
* an internet connection did not exist. This is because `vagrant-omnibus` 
  queries Rubygems.org to verify the desired version of Chef exists.
Create explicit tasks for supported provider acceptance tests
Add default, style and Travis-specific Rake tasks
Add Ruby 2.1.0 to the Travis test matrix.
Merge branch 'delay-config-validation'