Use canonical version of mail_room.

Gemfile

@@ -273,6 +273,6 @@ gem "newrelic_rpm"
 gem 'octokit', '3.7.0'
-gem "mail_room", github: "DouweM/mail_room", branch: "sidekiq"
+gem "mail_room", "~> 0.4.0"
 gem 'email_reply_parser'

Gemfile.lock

-GIT
-  remote: git://github.com/DouweM/mail_room.git
-  revision: c17c74cdef55dcd962333545fdb4d6e496cd9050
-  branch: sidekiq
-  specs:
-    mail_room (0.3.1)
-
 GEM
   remote: https://rubygems.org/
   specs:
@@ -379,6 +372,7 @@ GEM
       systemu (~> 2.6.2)
     mail (2.6.3)
       mime-types (>= 1.16, < 3)
+    mail_room (0.4.0)
     method_source (0.8.2)
     mime-types (1.25.1)
     mimemagic (0.3.0)
@@ -814,7 +808,7 @@ DEPENDENCIES
   jquery-ui-rails
   kaminari (~> 0.15.1)
   letter_opener
-  mail_room!
+  mail_room (~> 0.4.0)
   minitest (~> 5.3.0)
   mousetrap-rails
   mysql2