Sort development dependencies in gemspec
Remove hard dependency on Librarian-Chef

Only try to use the librarian gem if the Cheffile is found (and
`--no-librarian` is not specified). Complain to the user with
instructions if the gem is not found.

Also add a debug message is the Cheffile is not found.
Add unit tests for librarian gem detection
Try to load Librarian dependencies only if really needed
Lock down librarian dev dependency to v0.0.x

librarian v0.1.0 will not include librarian-chef any more,
as it will be extracted to own gem.
Clean up warning message of Librarian-Chef absence
Use `FileUtils.touch` to create empty Cheffiles on tests
Merge pull request #211 from tmatilai/librarian-dependency

Remove hard dependency on librarian