One of the notable changes in the upcoming Ruby 2.4 release is the unification of Fixnum and Bignum into a single Integer class. Following that change, the legacy classes were deprecated recently.
This pull request avoids triggering those deprecation warnings with a feature detection. If you maintain code that references those constants, you might want to adopt this pattern in your projects too!
Related to the unification of the integer classes: native extensions that references the rb_cFixum and rb_cBignum C constants need to be updated as well.
The popular json gem is one of those native extensions. Fortunately, all modern versions of Ruby bundles the json library. If your are targeting Ruby 1.9 and above, you could simply remove the json gem as a dependency.