BUG: Remove nonetype edges from bokeh module when not installed.
BUG: Remove nonetype edges from bokeh module when not installed.

When bokeh is not installed, it would create edges from `pd.DataFrame`
to `type(None)` that were invalid and would raise attribute errors.

cowlicks/odo,blaze/odo,quantopian/odo,cpcloud/odo,cowlicks/odo,ContinuumIO/odo,cpcloud/odo,quantopian/odo,blaze/odo,ContinuumIO/odo