Fix traceback when displaying chart with invalid spec

[jakevdp]

This addresses #480, with a workaround of the issue in ipython/ipython#11038

@ellisonbg – I'd appreciate your opinion on the approach I used here.

After this, the traceback in #480 looks like this:

import altair as alt
alt.Chart()

---------------------------------------------------------------------------
SchemaValidationError                     Traceback (most recent call last)
~/github/altair-viz/altair/altair/vegalite/v2/api.py in to_dict(self, *args, **kwargs)
    254         if dct is None:
    255             kwargs['validate'] = 'deep'
--> 256             dct = super(TopLevelMixin, copy).to_dict(*args, **kwargs)
    257 
    258         if is_top_level:

~/github/altair-viz/altair/altair/utils/schemapi.py in to_dict(self, validate, ignore, context)
    228                 self.validate(result)
    229             except jsonschema.ValidationError as err:
--> 230                 raise SchemaValidationError(self, err)
    231         return result
    232 

SchemaValidationError: Invalid specification

        altair.vegalite.v2.api.Chart, validating 'required'

        'mark' is a required property
        

[jakevdp]

Sorry, another commit snuck in. I'll update the base branch

[ellisonbg]

This isn't ideal, but I think we should do this for now.

[jakevdp]

I'm going to merge, with #568 open to track future fixes