Update doorkeeper to 4.2.0

Changelog: https://git.io/v6PnV

Gemfile

@@ -20,7 +20,7 @@ gem 'pg', '~> 0.18.2', group: :postgres
 
 # Authentication libraries
 gem 'devise',                 '~> 4.0'
-gem 'doorkeeper',             '~> 4.0'
+gem 'doorkeeper',             '~> 4.2.0'
 gem 'omniauth',               '~> 1.3.1'
 gem 'omniauth-auth0',         '~> 1.4.1'
 gem 'omniauth-azure-oauth2',  '~> 0.0.6'

Gemfile.lock

@@ -176,7 +176,7 @@ GEM
     diff-lcs (1.2.5)
     diffy (3.0.7)
     docile (1.1.5)
-    doorkeeper (4.0.0)
+    doorkeeper (4.2.0)
       railties (>= 4.2)
     dropzonejs-rails (0.7.2)
       rails (> 3.1)
@@ -836,7 +836,7 @@ DEPENDENCIES
   devise (~> 4.0)
   devise-two-factor (~> 3.0.0)
   diffy (~> 3.0.3)
-  doorkeeper (~> 4.0)
+  doorkeeper (~> 4.2.0)
   dropzonejs-rails (~> 0.7.1)
   email_reply_parser (~> 0.5.8)
   email_spec (~> 1.6.0)