Prevent JRuby rubygems warnings from breaking the build

/cc @myronmarston 
Nope, `RUBY_PLATFORM` is defined on all rubies, including 1.8.7.  That's why I recommend it, because you don't have to do the `defined?` dance.

```
➜ ruby -v -e "puts RUBY_PLATFORM"
ruby 1.8.7 (2013-06-27 patchlevel 374) [i686-darwin12.4.0]
i686-darwin12.4.0
```