Update json dependency to 1.8.5 for ruby >=2.4
Update json dependency to 1.8.5 for ruby >=2.4

Ruby 2.4 unified Fixnum and Bignum into Integer [1].  This causes issues
when running bundle, because bundler will fail when compiling native
extensions for json like so:

generator.c: In function ‘generate_json’:
generator.c:861:25: error: ‘rb_cFixnum’ undeclared (first use in this function); did you mean ‘mFixnum’?
  861 |     } else if (klass == rb_cFixnum) {
      |                         ^~~~~~~~~~
      |                         mFixnum

Setting the version of json to 1.8.5 includes the fix for this issue,
and allows etch server installability.

[1] https://bugs.ruby-lang.org/issues/12005

etch/etch,etch/etch,etch/etch,etch/etch,etch/etch