Renamed lib/Dnsruby to lib/dnsruby.

Ruby convention nowadays is to use snake case for directory and file names unless there is a compelling reason to do otherwise.  (The resource record files might be examples of \appropriate times to deviate from this standard.)

The recent pull request for dig-like to_s erroneously used lower case 'dnsruby', and when I tested the gem on Linux the test failed, since Linux case is strictly enforced.

Rather than fix this, I was hoping we could fix the directory name instead.  I'd like to do this for other directories and files too, but this one change touched a lot of files and I figured it might be good for this one to be a separate pull request.