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]
Add bug fix for mathn number formatting to changlog

[ci skip]
Port @myronmarston's in_sub_process helper
Add helper to run spec in subprocess with mathn loaded
Remove mathn require from formatters/helpers spec
Add spec for formatters/helpers mathn integration
Add mathn integration spec for html_formatter
Move MathnIntegrationSupport module to file in spec/support
Wrap specs in "produced HTML" descriptive block
Add whitespace to make setup vs. assertions distinct
DRY specs by wrapping body in helper method
Require support/in_sub_process for MathnIntegrationSupport
Merge pull request #1087 from iamvery/fix-formatter-issue-with-mathn-loaded

Use explicit conversion methods for stringified numbers