Add raco pkg show commands for debugging.

Background: The

     /usr/racket/bin/raco pkg install --deps search-auto --link markdown

apparently is _not_ installing the package deps in the Travis env,
even though it does so in my own Ubuntu 12.04 env.  WTF???

At least I assume that's why the subsequent

    /usr/racket/bin/raco test -x -p markdown

is failing with errors about not finding the package(s):

    markdown/test.rkt:3:9: rackjure/threading: standard-module-name-resolver: collection not found
      collection: "rackjure"
      in collection directories:
       /home/travis/build/greghendershott/markdown/markdown
       /home/travis/.racket/5.3.6/collects
       /usr/racket/collects

Use raco pkg show to confirm.