gnu: ruby-json-pure: Fix build.

The version of test-unit used is greater than one of the gemspec files was
specifying. It was already being changed in two places, so this commit changes
the json-java.gemspec file as well.

* gnu/packages/ruby.scm (ruby-json-pure)[arguments]: Add new
fix-json-java.gemspec phase.

gnu/packages/ruby.scm

@@ -3010,7 +3010,15 @@ a native C extension.")
            (lambda _
              ;; Regenerate gemspec so loosened dependency constraints are
              ;; propagated.
-             (invoke "rake" "gemspec"))))))
+             (invoke "rake" "gemspec")))
+         (add-after 'regenerate-gemspec 'fix-json-java.gemspec
+           (lambda _
+             ;; This gemspec doesn't look to be generated by the above
+             ;; command, so patch it separately.
+             (substitute* "json-java.gemspec"
+               (("%q<test-unit>\\.freeze, \\[\"~> 2\\.0\"\\]")
+                "%q<test-unit>.freeze, [\">= 2.0\"]"))
+             #t)))))
     (native-inputs
      `(("bundler" ,bundler)
        ("ragel" ,ragel)