Issue title: Fix lastIndexOf method
 Issue body: There is currently a method called `lastIndexOf` that lives in `/lib/lastIndexOf.js`.
It is incomplete and needs to be fixed!

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### Acceptance Criteria:
1. Running `yarn test lastIndexOf` 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](https://github.com/BlakeGuilloud/ganon/blob/master/CONTRIBUTING.md).


Thank you so much for your contribution!
 Comments: 
 Comment 0: Skeleton made in #317 
 Comment 1: @akram-rameez I'd like work on this (will be my second and last).
 Comment 2: Hey @mtso ! I need to update the README, but feel free to continue working on projects. I don't know why I put the rule in there that you should only do two.. As long as you are providing new methods for others, I don't see the harm in doing as many as you want. Going to mark this as in progress.
 Comment 3: Ok great :) @BlakeGuilloud

I think I made a mistake of pushing the solution to this into the master branch of my fork before my pull request was accepted, so the `lastIndexOf` commit is squashed in there.

I'll still need to make the skeleton for the next method, though.
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