Create a function that returns a specific member of the Fibonacci sequence:
A series of numbers in which each number ( Fibonacci number ) is the sum of the two preceding numbers. In this exercise, the Fibonacci sequence used is 1, 1, 2, 3, 5, 8, etc. To learn more about Fibonacci sequences, go to: https://en.wikipedia.org/wiki/Fibonacci_sequence
fibonacci(4); // returns the 4th member of the series: 3 (1, 1, 2, 3)
fibonacci(6); // returns 8