ggplot.to_html out of sync with latest changes?

Here is a small commit to update to_html in ggplot to support the removal of the notebook method from mpl. This should allow the original ggplot example notebook to work. The example should read:

import pandas
from bokeh.functional import *
df = pandas.read_csv("../tests/auto-mpg.csv")
g = ggplot(df, aes("displ", "mpg", color="green")) + facet_grid("cyl", "origin") + geom_point()
g.to_html(notebook=True)