Framework: Download api-request shim directly from GitHub

[aduth]

See: #1974 (comment)

This pull request seeks to download the api-request.js shim directly from GitHub, instead of through rawgit.com. The latter is necessary in cases where we would reference the URL by script tag directly, but given the behavior of vendor scripts to download the copy server-side to the plugin directory, this is not relevant for our usage.

Testing instructions:

Verify that the api-request.js shim continues to load as expected.

1. (Prerequisite) Your site must be running WordPress < 4.9-alpha
2. Delete your local copy if it exists: rm vendor/api-request.*
3. Navigate to Gutenberg > New Post
4. Note that the page loads without any errors

[mcsf]