stop using gulp.  switch to rollup.  This has some nice side effects - builds are very speedy, and we generate a single file for importing.  We also now generate both es6 and cjs bundles, and the latter is fixed such that you don't need to `require("libhoney").default` (though it still works. that'll go away the next major bump.)

stop using mocha.  switch to jest.

add prettier and reformat everything while we're here.

The only code changes should be lint fixes, prettier reformatting things, and the 9 extra lines at the end of `src/libhoney.js`