travis: add JSON gem for <= Sierra.

Closes #33503.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/travis.rb

@@ -55,6 +55,13 @@ class Travis < Formula
     sha256 "1e147d5d20f1ad5b0e23357070d1e6d0904ae9f71c3c49e0234cf682ae3c2b06"
   end
 
+  if MacOS.version <= :sierra
+    resource "json" do
+      url "https://rubygems.org/gems/json-2.1.0.gem"
+      sha256 "b76fd09b881088c6c64a12721a1528f2f747a1c2ee52fab4c1f60db8af946607"
+    end
+  end
+
   resource "launchy" do
     url "https://rubygems.org/gems/launchy-2.4.3.gem"
     sha256 "42f52ce12c6fe079bac8a804c66522a0eefe176b845a62df829defe0e37214a4"