Bump Sidekiq to 5.2.5

This reduces duplication caused by gitlab-monitor already shipping
Sidekiq 5.2.5. The only changes appear to be minor:

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

Gemfile.lock

@@ -627,7 +627,7 @@ GEM
       httpclient (>= 2.4)
       multi_json (>= 1.3.6)
       rack (>= 1.1)
-    rack-protection (2.0.4)
+    rack-protection (2.0.5)
       rack
     rack-proxy (0.6.0)
       rack
@@ -823,8 +823,9 @@ GEM
       rack
     shoulda-matchers (3.1.2)
       activesupport (>= 4.0.0)
-    sidekiq (5.2.3)
+    sidekiq (5.2.5)
       connection_pool (~> 2.2, >= 2.2.2)
+      rack (>= 1.5.0)
       rack-protection (>= 1.5.0)
       redis (>= 3.3.5, < 5)
     sidekiq-cron (1.0.4)