Reduce the size of the caniuse data.

Summary:
Since we're using `prefixAll` and don't really care about the browser
versions (for now), I made the caniuse data smaller to match what we're
actually using it for.

This reduces the gzipped size of prefixer.min.js to 4316 bytes.

Test Plan: - `npm run test`

Reviewers: jlfwong

Reviewed By: jlfwong

Differential Revision: https://phabricator.khanacademy.org/D25207

Author: xymostech