Require platform version option

Maintain compatibility with ChefSpec, but otherwise always require
`platform` and `version` options (or `path`).

Continuation for cleanup in #25.
Remove dead code in #platform_path

The exception was never raised as there was a `File.exists?` missing.
But after #7 we anyway want to look for the Github repo before throwing
the exception. But improve the error message to include the platform.
Merge pull request #26 from tmatilai/require_platform_version

Require platform version option