Upgrade tfjs from 0.11 to 0.12 in examples

[caisq]

Also change ^ to ~ in the version strings, so that upgrades like this won't be absolutely necessary in the future.

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This change is 

[nsthorat]

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Review status: complete! 1 of 1 LGTMs obtained

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addition-rnn/package.json, line 13 at r1 (raw file):

  "dependencies": {
    "@tensorflow/tfjs": "~0.12.0",
    "vega-embed": "^3.0.0"

can you update these to ~ while you are here?

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Comments from Reviewable

[dsmilkov]

I made them ^ so that we are "forced" to keep our examples up to date with our API. Using ~ will take the examples much more stable, but then we can be lazy about upgrading them

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Review status: complete! 1 of 1 LGTMs obtained

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Comments from Reviewable

[caisq]

Done switching back to ^

Reviewable status: complete! 1 of 1 LGTMs obtained