Don’t require("crypto") in browsers.

[mbostock]

Same fix as tensorflow/tfjs#355.

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This change is 

[dsmilkov]

Thanks! If crypto is ignored here, then users of tfjs-core (e.g. tfjs) won't have to explicitly ignore it, right?

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Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[nsthorat]

Awesome, thanks Mike! I think you're right, since we ignore it we don't have to do it for all dependencies of core.