Update Fibonacci solution, test to focus on arrays

Previous Fibonacci solution didn't use arrays, and since this is an
array-heavy section, it seemed better to have that be the method here.

Also, it accounts for entering 0 as an argument without having to add
any extra code, which takes care of some currently open issues.
Issues #192 and #236 are about the same thing.

I added a test for that as well.

10_fibonacci/solution/fibonacci-solution.spec.js

@@ -16,6 +16,9 @@ describe('fibonacci', () => {
   test('25th fibonacci number is 75025', () => {
     expect(fibonacci(25)).toBe(75025);
   });
+  test('0th fibonacci number is o', () => {
+    expect(fibonacci(0)).toBe(0);
+  });
   test("doesn't accept negatives", () => {
     expect(fibonacci(-25)).toBe('OOPS');
   });