Add a missing fastboot dependencies whitelist entry.

[cvx]

Fixes an error (introduced in #112) that occurs whenever a fastboot-enabled app that uses ember-fetch tries to handle a request:

There was an error running your app in fastboot. More info about the error:
 Error: Unable to require module 'abortcontroller-polyfill/dist/cjs-ponyfill' because it was not in the whitelist.
    at Object.require (/app/node_modules/fastboot/src/ember-app.js:133:15)
    at Module.callback (/app/tmp/broccoli_merge_trees-output_path-76mgNO0Y.tmp/ember-fetch/fastboot-fetch.js:4:44)

[xg-wang]

Should be able to delete this line

[cvx]

You're right. Changed, tested, and updated the PR. Thanks!