Added some changes covering Mathias's commentaries.

IPython/nbconvert/exporters/reveal.py

@@ -55,10 +55,6 b' class RevealExporter(BasicHTMLExporter):'
                 },
             'RevealHelpTransformer':{
                 'enabled':True,
-                'url_prefix':'//cdn.jsdelivr.net/reveal.js/2.4.0',
-                # If you want to use a local reveal.js library, just
-                # comment the previous line and uncomment the next one.
-                #'url_prefix':'reveal.js',
                 },                
             })
         c.merge(super(RevealExporter,self).default_config)

IPython/nbconvert/transformers/revealhelp.py

@@ -13,7 +13,7 b''
 #-----------------------------------------------------------------------------
 
 from .base import ConfigurableTransformer
-from IPython.utils.traitlets import Bytes
+from IPython.utils.traitlets import Unicode
 
 #-----------------------------------------------------------------------------
 # Classes and functions
@@ -21,7 +21,10 b' from IPython.utils.traitlets import Bytes'
 
 class RevealHelpTransformer(ConfigurableTransformer):
 
-    url_prefix = Bytes("", config=True, help="url prefix to get reveal.js files")
+    url_prefix = Unicode('//cdn.jsdelivr.net/reveal.js/2.4.0',
+                         config=True,
+                         help="""If you want to use a local reveal.js library,
+                         use 'url_prefix':'reveal.js' in your config object.""")
 
     def call(self, nb, resources):
         """
@@ -56,4 +59,3 b' class RevealHelpTransformer(ConfigurableTransformer):'
         resources['reveal']['url_prefix'] = self.url_prefix            
                     
         return nb, resources
-    
  No newline at end of file