Bump doorkeeper to ~> 2.2.0

Closes #2746

Gemfile

@@ -16,7 +16,7 @@ gem "pg", '~> 0.18.2', group: :postgres
 # Authentication libraries
 gem 'devise',                 '~> 3.5.2'
 gem 'devise-async',           '~> 0.9.0'
-gem 'doorkeeper',             '~> 2.1.3'
+gem 'doorkeeper',             '~> 2.2.0'
 gem 'omniauth',               '~> 1.2.2'
 gem 'omniauth-bitbucket',     '~> 0.0.2'
 gem 'omniauth-facebook',      '~> 3.0.0'

Gemfile.lock

@@ -164,7 +164,7 @@ GEM
     docile (1.1.5)
     domain_name (0.5.24)
       unf (>= 0.0.5, < 1.0.0)
-    doorkeeper (2.1.4)
+    doorkeeper (2.2.2)
       railties (>= 3.2)
     dropzonejs-rails (0.7.1)
       rails (> 3.1)
@@ -824,7 +824,7 @@ DEPENDENCIES
   devise-async (~> 0.9.0)
   devise-two-factor (~> 2.0.0)
   diffy (~> 3.0.3)
-  doorkeeper (~> 2.1.3)
+  doorkeeper (~> 2.2.0)
   dropzonejs-rails (~> 0.7.1)
   email_reply_parser (~> 0.5.8)
   email_spec (~> 1.6.0)