Fix bug in lcm() function

Hi! "Least common multiple" function throwed following error to me:
```
/Library/Ruby/Gems/2.0.0/gems/sassy-fractions-1.0/lib/sassy-fractions.rb:39:in `/': Sass::Script::Value::Number can't be coerced into Fixnum

`.to_i` method was missing on line 39 on gcd() call

It now works as expected!