resolve `latest` to a real Chef version; fixes GH-2

Queries the RubyGems.org's Ruby API to determine the latest version of
Chef.

The `InstallChef` action does not install the Omnibus Chef package if
the requested version is already installed. Resolving a version of
`latest` to a real version (i.e. 11.4.0) allows this check to succeed
when appropriate.
validate user provided Chef version; fixes GH-4

Queries RubyGems.org's Ruby API to see if the user-provided Chef version
is in fact a real Chef version!
retrieve omnibus.chef_version directly from global config

This ensures we are dealing with the final, finalized version.
use `:latest` in virtualbox acceptance test
update to latest vagrant (1.2.1) and vagrant-aws (0.2.1)
add Apache 2.0 license headers
v1.0.1