Fix promisify function

[kylevv]

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

Promisify takes an asynchronous function that uses node style callbacks and returns a function that will return a promise instead.

Acceptance Criteria:

1. Running yarn test promisify results in tests passing.
2. You have written a skeleton method for someone else to work on.
3. You have written tests surrounding your skeleton method.
4. Running yarn lint does not print any errors to the console!
5. Optional: write one or two more tests surrounding the method you are solving to account for potential edge cases.

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! A pull request will most likely be denied if it does not contain a skeleton method for someone else to work on! For more information, please read the Contributing Guide.

Thank you so much for your contribution!

[kylevv]

Requires #717 to be merged first.

[BlakeGuilloud]

Can't believe no one has taken this yet!

[arturoromeroslc]

I can take a stab at it.

[ktilcu]

@arturoromeroslc Have fun!