Upgrade Doorkeeper from 2.2.2 to 3.1.0

I’d rather upgrade one major version at a time, so Doorkeeper needs to
be upgraded to 3.x before it can be upgraded to 4.x (which includes
Rails 5 support).

Changelog:
https://github.com/doorkeeper-gem/doorkeeper/blob/master/NEWS.md#310

Gemfile

@@ -19,7 +19,7 @@ gem "pg", '~> 0.18.2', group: :postgres
 
 # Authentication libraries
 gem 'devise',                 '~> 3.5.4'
-gem 'doorkeeper',             '~> 2.2.0'
+gem 'doorkeeper',             '~> 3.1'
 gem 'omniauth',               '~> 1.3.1'
 gem 'omniauth-auth0',         '~> 1.4.1'
 gem 'omniauth-azure-oauth2',  '~> 0.0.6'

Gemfile.lock

@@ -173,7 +173,7 @@ GEM
     diff-lcs (1.2.5)
     diffy (3.0.7)
     docile (1.1.5)
-    doorkeeper (2.2.2)
+    doorkeeper (3.1.0)
       railties (>= 3.2)
     dropzonejs-rails (0.7.2)
       rails (> 3.1)
@@ -922,7 +922,7 @@ DEPENDENCIES
   devise (~> 3.5.4)
   devise-two-factor (~> 2.0.0)
   diffy (~> 3.0.3)
-  doorkeeper (~> 2.2.0)
+  doorkeeper (~> 3.1)
   dropzonejs-rails (~> 0.7.1)
   email_reply_parser (~> 0.5.8)
   email_spec (~> 1.6.0)