Fix for Rubinius

Problem
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Currently, CI is failing b/c singleton_class? was added to MRI2.1 but not Rbx. e.g. https://travis-ci.org/pry/pry/builds/32133642

I did not look into why it is suddenly failing now. For quite a while, Rbx builds were erroring out (e.g.  https://travis-ci.org/pry/pry/jobs/30421343) They are no longer erroring, but are now failing, for the above reason.

Context
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Rbx has recently (11 days ago, 30 July 2014) received this method https://github.com/rubinius/rubinius/commit/2e71722dba53d1a92c54d5e3968d64d1042486fe

But the functionality has been available since April 2011 https://github.com/rubinius/rubinius/commit/4310f6b2ef3c8fc88135affe697db4e29e4621c4

Solution
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Fix the failures by falling back to the older functionality if we are on Rubinius, and the new method is unavailable.