[build-troubleshooting] Explicitly calling out need for rake clean

* Changing in response to:
  https://travis-ci.org/jedcn/reveal-ck/jobs/4664577

Which says:

/home/travis/build/jedcn/reveal-ck/rakelib/slides.rake:2:in
`<top (required)>': uninitialized constant Object::CLEAN (NameError)
/home/travis/build/jedcn/reveal-ck/bin/reveal-ck:10:in `load'
/home/travis/build/jedcn/reveal-ck/bin/reveal-ck:10:in `block in <main>'
/home/travis/build/jedcn/reveal-ck/bin/reveal-ck:9:in `each'