Fix return type of parse

[TrueWill]

Fixes https://github.com/i18next/i18next-http-backend/issues/75

I'm not clear on how to write a test for an incorrect typedef.

Checklist

• [x] only relevant code is changed (make a diff before you submit the PR)
• [ ] run tests npm run test
• [ ] tests are included
• [ ] documentation is changed or added

[adrai]

@pedrodurek ok for you?