Oh, you're right. I forgot the version constraint. I thought you meant a Ruby version constraint (e.g. `if RUBY_VERSION =~ /1.8.7/`). This was my actual code, which does the right thing on both Ruby 1.8 and 1.9 (and 2.0 and 2.1):
```ruby
gem 'mime-types', '~> 1.25', :platforms => :ruby_18
```