Bump Sidekiq and other related gems

* Sidekiq changes:
  https://github.com/mperham/sidekiq/blob/master/Changes.md

* Rack 1.6.1 fixes CVE-2014-9490:
  https://github.com/rack/rack/blob/master/CHANGELOG.md

* rack-protection 2.0.4 only fixes a minor message:
  https://github.com/sinatra/sinatra/commits/master/rack-protection

Gemfile.lock

@@ -596,7 +596,7 @@ GEM
       get_process_mem (~> 0.2)
       puma (>= 2.7, < 4)
     pyu-ruby-sasl (0.0.3.3)
-    rack (1.6.10)
+    rack (1.6.11)
     rack-accept (0.4.5)
       rack (>= 0.4)
     rack-attack (4.4.1)
@@ -608,7 +608,7 @@ GEM
       httpclient (>= 2.4)
       multi_json (>= 1.3.6)
       rack (>= 1.1)
-    rack-protection (2.0.3)
+    rack-protection (2.0.4)
       rack
     rack-proxy (0.6.0)
       rack
@@ -802,7 +802,7 @@ GEM
       rack
     shoulda-matchers (3.1.2)
       activesupport (>= 4.0.0)
-    sidekiq (5.2.1)
+    sidekiq (5.2.3)
       connection_pool (~> 2.2, >= 2.2.2)
       rack-protection (>= 1.5.0)
       redis (>= 3.3.5, < 5)