update Gemfile syntax to work with Ruby oddity

Now I believe it does what was intended.

Fun times with Ruby

```
justin$ bundle exec ruby -e 'puts (RUBY_PLATFORM=~ /win32/).class'
NilClass
justin$ bundle exec ruby -e 'puts (!RUBY_PLATFORM=~ /win32/).class'
NilClass
justin$ bundle exec ruby -e 'puts (!!RUBY_PLATFORM=~ /win32/).class'
NilClass
justin$ bundle exec ruby -e 'puts (not RUBY_PLATFORM=~ /win32/).class'
TrueClass
```