I've tried `platforms` two ways:

```ruby
  platforms :ruby_18 do
    gem "sanitize", ">= 2", "< 2.0.4", :require => false
  end

  platforms :ruby_19, :ruby_20 do
    gem "sanitize", "~> 2.0", :require => false
  end
```

```ruby
  gem "sanitize", ">= 2", "< 2.0.4", :require => false, :platforms => :ruby_18
  gem "sanitize", "~> 2.0",          :require => false, :platforms => [:ruby_19, :ruby_20]
```

Bundler doesn't like either way.

```
❯ bundle install
You cannot specify the same gem twice with different version requirements. 
You specified: sanitize (< 2.0.4, >= 2) and sanitize (~> 2.0)
```

I think we should stick with the `RUBY_VERSION` conditional.  Thoughts?