redmine: fix compilation with ruby 2.3

Just bumped the JSON dependency manually to the one bundled with ruby 2.3

pkgs/applications/version-management/redmine/Gemfile.lock

···
     jquery-rails (2.0.3)
       railties (>= 3.1.0, < 5.0)
       thor (~> 0.14)
-    json (1.8.1)
+    json (1.8.3)
     mail (2.5.4)
       mime-types (~> 1.16)
       treetop (~> 1.4.8)

pkgs/applications/version-management/redmine/Gemfile.nix

···
 }
 {
 name = "json";
-hash = "961bfbbfa9fda1e857e9c791e964e6664e0d43bf687b19669dfbc7cdbc5e0200";
-url = "http://rubygems.org/downloads/json-1.8.1.gem";
-version = "1.8.1";
+hash = "1nsby6ry8l9xg3yw4adlhk2pnc7i0h0rznvcss4vk3v74qg0k8lc";
+url = "http://rubygems.org/downloads/json-1.8.3.gem";
+version = "1.8.3";
 }
 {
 name = "mail";