change default for slides to direct to the cdn rather than locally, q…

…uick fix for jupyter#702

nbconvert/exporters/slides.py

@@ -90,7 +90,7 @@ def _reveal_url_prefix_default(self):
             warn("Please update RevealHelpPreprocessor.url_prefix to "
                  "SlidesExporter.reveal_url_prefix in config files.")
             return self.config.RevealHelpPreprocessor.url_prefix
-        return 'reveal.js'
+        return 'https://cdnjs.cloudflare.com/ajax/libs/reveal.js/3.1.0'
 
     reveal_theme = Unicode('simple',
         help="""