Load graph reference from pkg copy of plot schema

[etpinard]

fixes #613

[etpinard]

PROS

• import plotly will never break because of plot schema bugs e.g.Fix defs.metaKeys field plotly.js#1182
• import plotly will never make a request to plot.ly

CONS

• plotly.plotly (i.e. users making graph via plot.ly) won't be relying in the correct plot schema for key validation. For example, if we add an attribute to plotly.js and then to plot.ly, plotly.plotly users trying to user this attribute will get a validation error until we bump the default-schema.json during plotly.offline plotly.js version bumps.

I personally think the item under CONS isn't a big deal because:

• We should update the plotly.offline plotly.js frequently
• If a user really want to use bleeding edge keys, they can always turn off validation

[etpinard]

cc @theengineear @chriddyp @cldougl @Kully

[theengineear]

@etpinard I'm shifting things around wrt our api for a large-ish release. I like the idea of no longer auto-downloading the plot-schema. We have a very simple way to update the plot schema and can just do that with minor version bumps if desired.

@chriddyp and @Kully, ya'll OK with stopping the auto-download of plot-schema as well?

[chriddyp]

I'm OK with it!

[theengineear]

The ideas from this have been implemented. Closing.