Remove 'request' dependency

[donmccurdy]

Indirect dependency "request" (ndarray-pixels → get-pixels → request) is deprecated, and its own dependencies are drifting out of date. See:

• SECURITY: npm audit found an issue in request dep. scijs/get-pixels#62
• Replace request lib? scijs/get-pixels#44

If this can't be patched in get-pixels, I may need to eventually consider a fork, or bringing that implementation into this repository. I don't need GIF support here, so (with a major version bump) some size could be saved by doing so. (This doesn't apply, as I'm only using get-pixels in Node.js environments, and letting the browser handle things otherwise.)