feat: support AbortController signal

[x] feat: support AbortController signal
[x] chore(deps): update dependencies
[x] ci: add node 16 to test matrix & remove travis config

const { fetch } = require('gofer');
const ac = new AbortController(); // Node 15+

ac.signal.addEventListener('abort', () => console.log('Aborted!'), { once: true });

await Promise.race([
  fetch('https://example.com', { signal: ac.signal }).text().catch(err => /*  err this will say it was aborted */),
  async () => { return '<html>'; }
]).finally(() => ac.abort());

groupon / gofer

feat: support AbortController signal #125