Fix prime function

[BlakeGuilloud]

There is currently a method called prime that lives in /lib/prime.js.
It is incomplete and needs to be fixed!

Acceptance Criteria:

1. Running yarn test prime results in tests passing.

Please include the skeleton of a new method + an accompanying test for someone else to work on at the time of creating a pull request!

[Phlegz]

@BlakeGuilloud Hi :) Is there a way that I could add an "in progress" label to any of the open issues so nobody else would work on it at the same time? Thanks.

[BlakeGuilloud]

Nice wokr @Phlegz ! Closing this out.