Several slightly advance usage of Ruby

Demonstrating these Ruby concepts:
* range
* Enumerable#inject
* "Opening" existing class
* Hiding test suite in conditional

And, replacing TABs with double-spaces.
Another version, though a bit more brain melty:

```
def num_to_fizzbuzz(num)
  return num unless num % 5 == 0 || num % 3 == 0
  result = ""
  result += "fizz" if num % 3 == 0
  result += "buzz" if num % 5 == 0
  result
end

def fizzbuzz(up_to)
  (1..up_to).map do |number|
    num_to_fizzbuzz(number)
  end
end
```
This:
  * Creates a very small function `num_to_fizzbuzz` that, given a number, returns either the number, "fizz", "buzz" or "fizzbuzz"
  * [Maps](http://www.ruby-doc.org/core-2.1.2/Array.html#method-i-map) the [range](http://www.ruby-doc.org/core-2.1.2/Range.html) from numbers through `num_to_fizzbuzz`