Suggestions for class 4 old school roman numerals

`rom_num` was being reassigned for every iteration of the `while` loop. In other words, every time something was pushed onto the array, it was erased by the reassignment.

There solution in this pull request fixes your issue, but doesn't solve the exercise 100%. Check out what the book has to say about the `join` method on section 8.2 (page 55).

---

I also made a few stylistic improvements. 

Never put spaces between a method and it's parenthesis:

```ruby
# bad
some_method (an_argument)

# good
some_method(an_argument)
```

Never put spaces between an array and it's parenthesis:

```ruby
# bad
some_array [index]

# good
some_array[index]
```