Isochronous v4.0.0-alpha.3

• Tidy .gitignore. Closes #50.
• Build with Node.js 10 at Travis CI. Closes #47.
• Do not upgrade npm during Travis CI build. Closes #49.
• Run under NYC at Travis CI. Closes #48.
• Remove TODO covered by async/await conversion. See #33.
• Remove Extant dependency. Closes #46.
• Remove delay from unit tests. See #45.
• Replace Delay with setTimeout. Closes #45.
• Revert tests to Proof. Closes #44.

Closes #51.