[Features] Update tensorflowjs dependency

[WenheLI]

As the latest tensorflowjs provide webworker support, it makes the inference less likely to block the UI rendering (in another world, it will be less likely to freeze the UI).
Here is a post that describes the webworker in tensorflowjs.
I think we should upgrade the tensorflowjs so that our ml5 could have such support too.
I can also write a quick tutorial for webworker in ml5 after the upgrade.

[joeyklee]

@WenheLI - This is definitely on our list, however since we have tfjs dependencies that vary across the different models we support, we should try to stay in-sync with those if possible. As you've pointed out, there's a lot of exciting updates from the tfjs side so we should definitely try to bump the version up when we can!

[bomanimc]

I think we were able to accomplish this in #846, though there's still work to be done around updating all of our other models to their latest versions.

@WenheLI would you still be interested in the webworker changes?

[WenheLI]

@bomanimc - Sure! I can add some examples of how to combine webworker with ml5!

[bomanimc]

Closing this issue out after the update in #846. @WenheLI please feel free to make a separate issue for the web worker changes if you'd like!