It's quite long time, since we unbundled the gems shipped by default
with Ruby. It caused some pains, but it was working more or less quite
OK. Unfortunately, with release of RoR 5.0 and this PR  (which is
just last in a row), Rails does not explicitly specify the dependency on
JSON gem anymore and therefore, if you are using Bundler, it cannot load
the unbundled rubygem-json, although it is available on your system.
Although I am pretty sad, that the Rails developers decided to not list
their dependencies anymore (and I wholehartly agree with the first part
of ), I introduced the symlinks into rubygem-json, which will make
the Rails 5.0 work without specifying JSON gem in the Gemfile. Please
test the new releases of rubygem-json [3, 4] to make sure I did not
PS. I'be very glad if you can always specify all the dependecies of your
application in your Gemfile or in your .gemspec, being bundled by
default with Ruby or not. If nothing else, it is good habit ...