Upgrade Sidekiq from 4.1.4 to 4.2.1, remove dependency on Sinatra

Changelog:
https://github.com/mperham/sidekiq/blob/921e939f995fbb5238975d4121d728b95be99ab5/Changes.md#421

Sinatra is no longer required and sidekiq uses a vanilla Rack app for
its Web UI now.

Gemfile

@@ -135,8 +135,7 @@ gem 'after_commit_queue', '~> 1.3.0'
 gem 'acts-as-taggable-on', '~> 3.4'
 
 # Background jobs
-gem 'sinatra', '~> 1.4.4', require: false
-gem 'sidekiq', '~> 4.0'
+gem 'sidekiq', '~> 4.2'
 gem 'sidekiq-cron', '~> 0.4.0'
 gem 'redis-namespace', '~> 1.5.2'
 

Gemfile.lock

@@ -672,11 +672,11 @@ GEM
       rack
     shoulda-matchers (2.8.0)
       activesupport (>= 3.0.0)
-    sidekiq (4.1.4)
+    sidekiq (4.2.1)
       concurrent-ruby (~> 1.0)
       connection_pool (~> 2.2, >= 2.2.0)
+      rack-protection (~> 1.5)
       redis (~> 3.2, >= 3.2.1)
-      sinatra (>= 1.4.7)
     sidekiq-cron (0.4.0)
       redis-namespace (>= 1.5.2)
       rufus-scheduler (>= 2.0.24)
@@ -686,10 +686,6 @@ GEM
       json (>= 1.8, < 3)
       simplecov-html (~> 0.10.0)
     simplecov-html (0.10.0)
-    sinatra (1.4.7)
-      rack (~> 1.5)
-      rack-protection (~> 1.4)
-      tilt (>= 1.3, < 3)
     slack-notifier (1.2.1)
     slop (3.6.0)
     spinach (0.8.10)
@@ -958,10 +954,9 @@ DEPENDENCIES
   settingslogic (~> 2.0.9)
   sham_rack (~> 1.3.6)
   shoulda-matchers (~> 2.8.0)
-  sidekiq (~> 4.0)
+  sidekiq (~> 4.2)
   sidekiq-cron (~> 0.4.0)
   simplecov (= 0.12.0)
-  sinatra (~> 1.4.4)
   slack-notifier (~> 1.2.0)
   spinach-rails (~> 0.2.1)
   spinach-rerun-reporter (~> 0.0.2)
@@ -993,4 +988,4 @@ DEPENDENCIES
   wikicloth (= 0.8.1)
 
 BUNDLED WITH
-   1.12.5
+   1.13.0