Remove pdf from display order for HTML export

Backport of jupyter/nbconvert#5

Fixes issues with graphs in R kernel notebooks (see
IRkernel/IRkernel#145 )

IPython/nbconvert/exporters/html.py

@@ -1,57 +1,65 b''
 """HTML Exporter class"""
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 import os
 
 from IPython.nbconvert.filters.highlight import Highlight2HTML
 from IPython.config import Config
 
 from .templateexporter import TemplateExporter
 
 #-----------------------------------------------------------------------------
 # Classes
 #-----------------------------------------------------------------------------
 
 class HTMLExporter(TemplateExporter):
     """
     Exports a basic HTML document.  This exporter assists with the export of
     HTML.  Inherit from it if you are writing your own HTML template and need
     custom preprocessors/filters.  If you don't need custom preprocessors/
     filters, just change the 'template_file' config option.  
     """
     
     def _file_extension_default(self):
         return '.html'
 
     def _default_template_path_default(self):
         return os.path.join("..", "templates", "html")
 
     def _template_file_default(self):
         return 'full'
     
     output_mimetype = 'text/html'
     
     @property
     def default_config(self):
         c = Config({
             'NbConvertBase': {
-                'display_data_priority' : ['application/javascript', 'text/html', 'text/markdown', 'application/pdf', 'image/svg+xml', 'text/latex', 'image/png', 'image/jpeg', 'text/plain']
+                'display_data_priority' : ['application/javascript',
+                                           'text/html',
+                                           'text/markdown',
+                                           'image/svg+xml',
+                                           'text/latex',
+                                           'image/png',
+                                           'image/jpeg',
+                                           'text/plain'
+                                          ]
                 },
             'CSSHTMLHeaderPreprocessor':{
                 'enabled':True
                 },
             'HighlightMagicsPreprocessor': {
                 'enabled':True
                 }
             })
         c.merge(super(HTMLExporter,self).default_config)
         return c
 
     def from_notebook_node(self, nb, resources=None, **kw):
         langinfo = nb.metadata.get('language_info', {})
         lexer = langinfo.get('pygments_lexer', langinfo.get('name', None))
         self.register_filter('highlight_code',
                              Highlight2HTML(pygments_lexer=lexer, parent=self))
         return super(HTMLExporter, self).from_notebook_node(nb, resources, **kw)