bump redis-namespace even though the security issue does not affect us

The issue only shows if you have a send :exec, user_input in your
codebase, neither we nor Sidekiq does that.

See resque/redis-namespace#64 and
resque/redis-namespace#65 (comment)

Gemfile.lock

@@ -326,7 +326,7 @@ GEM
       json (~> 1.4)
     redcarpet (3.0.0)
     redis (3.0.4)
-    redis-namespace (1.3.0)
+    redis-namespace (1.3.1)
       redis (~> 3.0.0)
     remotipart (1.2.1)
     rmagick (2.13.2)