Fixes Bokeh Logo repetition in nbviewer notebooks.

Resolves https://github.com/ContinuumIO/bokeh/issues/809
Hey guys, sorry for the delayed response.

I would say that in web dev this would be the correct solution (providing graceful degradation if the user is for some reason disabling css, so they at least see a text replacement for the logo).  However, having asked a couple people who use notebooks , it sounds like it's very unlikely a user would ever disable css?  

And as Evan mentioned, removing the self-closing tag is sufficient to eliminate the bug.