Revert ruby-jwt version (#5575)

jwt 2.1.0 still does not work well.
ref. https://github.com/zaru/webpush/issues/42

Gemfile.lock

@@ -236,7 +236,7 @@ GEM
       multi_json (~> 1.11)
       rdf (~> 2.2)
     jsonapi-renderer (0.1.3)
-    jwt (2.1.0)
+    jwt (1.5.6)
     kaminari (1.1.1)
       activesupport (>= 4.1.0)
       kaminari-actionview (= 1.1.1)