Call out rubocop for static code analysis

Rubocop was introduced to Travis CI back in February but the README was
not updated.  Given a rubocop offense will fail the build, it seems it
should be highlighted along with tests and api specs.
Nginx waits for requests to complete before shutting down

* Drain script asks nginx to shutdown gracefully
* Monit kills nginx hard because if monit is killing the process, the drain script has already run and failed.

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>
Merge pull request #191 from sykesm/patch-1

Call out rubocop for static code analysis
Fix rubocop violation

bosh-templates/nginx_drain.rb:24:8: W: Ambiguous negative number operator
Add :rubocop rake task, run as a dependency of :spec task

* update travis config to use task
Rake tasks should not blow up if rubocop is not loaded