Install JavaScript dependencies on bin/update

[zenizh]

Summary

refs c873746

On above commit aimed to comment out the calling of bin/yarn but it was removed from bin/update in spite of remaining in bin/setup.

I think it is helpful to us who are using yarn if generated bin/update has this code.

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[javan]

r? @dhh

[javan]

Is this intended to be commented out?

[dhh]

Yes, because even though we install yarn by default, it's not yet every app that actually needs to bundle JS assets every time. If we go all in on Webpacker for JS, that would change.

[javan]

Question is: do we want to merge this change? Is bin/update intended to be modified after creation? If not, I don't see the benefit of adding commented out code.

Perhaps bin/yarn could run if package.json is present?

[zenizh]

Is bin/update intended to be modified after creation?

Currently at least bin/setup is intended to be modified after creation.
For consistency I think we should add this commented out code to bin/update or remove the same code from bin/setup.
https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/app/templates/bin/setup.tt#L18-L21

And this code is helpful for us who are using yarn so I think this should be kept.