Update to stable ipywidgets/jupyter-js-widgets when available

[parente]

Stop cloning from github in dockerfiles. Update package.json / bower.json to use the new version.

[parente]

When jupyter-widgets/ipywidgets#336 lands and the modules are released on npm, we will want to give them a shot.

• ipywidgets python package in kernel gateway image
• jupyter-js-widgets in package.json for dashboard server
• hopefully, ignore jupyter-js-widgets-nbextension

[parente]

Removing milestone. ipywidgets 5.0 is WIP and the npm releases of jupyter-js-widgets are still a bit in flux. We might want to bump to the latest and greatest in git master to ensure we're still compatible, though.

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@jhpedemonte says we need to take a look at the latest and make sure we're still compatible as 1.0.0b comes out.

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Retargeting for 0.4.0. Work is already going into decl widgets to support this and it cleans us up over here from having to do custom builds of the jupyter-js-widgets.

[parente]

Closed by PR #153