Fix broken build process - updated through2 dep to v3.0.1

[joeyklee]

Hi!

What change does this produce

• Fix broken build process - updated through2 dep to v3.0.1

Does this cause any breaking changes

• No

Any other notes

• Make sure to merge in the following PRs associated with the magicbook-katex and magicbook-codesplit which are also using an old version of through2 (in progress)

cc/ @shiffman

[runemadsen]

Thank you!