Add "Pkg.add" to our Julia test to ensure it continues functioning properly

test/tests/julia-hello-world/container.jl

@@ -2,3 +2,6 @@ println("Hello World!");
 
 # https://github.com/docker-library/julia/pull/6
 download("https://google.com");
+
+# https://github.com/docker-library/julia/pull/9
+Pkg.add("JSON");