Use ChefDK's path to compute the SemVer compliant version

This change has a few advantages:

* Removes `git describe` shell-out logic that already exists in 
  `Omnibus::BuildVersion`.
* Ensures the build version generated respects any timestamp related 
  flags passed to the `omnibus build` command.
take full advantage of git cache by building `chef` last
Only always build chef and chefdk in their projects
Compute build_version in the build block

This ensures the project.build_version a component might have overridden 
is properly reflected in the project's `version-manifest.txt` file.
Merge pull request #164 from opscode/schisamo/use-omnibus-buildversion

Use Omnibus::BuildVersion to generate build version