fix: remove unnecessary async keyword

[danielkcz]

What did you implement:

I've removed the async keyword to make the loader compatible with cosmiconfigSync().

How did you implement it:

See above :)

How can we verify it:

Try to use the plugin with cosmiconfigSync().

Tasks:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[jacksteamdev]

@FredyC ESLint require-await is no joke! 🤦‍♂️

If @hassankhan isn't maintaining this anymore, have you thought about publishing this on NPM?

[hassankhan]

Hi @jacksteamdev @FredyC 👋

Unfortunately I don't have commit rights to this repo as I'm not an @EndemolShineGroup employee any more. I've been meaning to fork the repo and republish it under @radicalcondor, but I didn't have a firm deadline for it 😅

[hassankhan]

Just got my commit privileges restored, thanks for the PR @FredyC 🎉

[hassankhan]

Ahh just realised that Travis CI doesn't work any more, going to need to switch over to GitHub Actions before the Ci can release a new version. I've created a new issue for that: #185