Create fibonacci_sequence.rb

Working on Project Euler, Problem #2:

Each new term in the Fibonacci sequence is generated by 
adding the previous two terms. By starting with 1 and 2, 
the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence whose 
values do not exceed four million, find the sum of the 
even-valued terms.