OS: use RbConfig instead of RUBY_PLATFORM

[bfontaine]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew tests with your changes locally?

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RUBY_PLATFORM is always "java" when running JRuby, no matter what the underlying platform is. Using RbConfig fixes that (see pry/pry#386).

[MikeMcQuaid]

Thanks @bfontaine!