Use explicit conversion methods for stringified numbers

* As @greggroth pointed out in rspec/rspec-core#1086, the formatter
  does not work as expected when the mathn lib is loaded. This lib adds
  the Rational type which is formatted as "n/n" when stringified rather
  than the float form. By explicitly converting the arithmetic to the
  types we wish to stringify, this display is corrected.

[#1086]

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The concern @greggroth and I have is now the spec suite is running w/ mathn loaded which is most likely not going to be the use case of many end users. How even do we isolate this spec?