fix: Gemfile & Gemfile.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-DOORKEEPER-5702995
- https://snyk.io/vuln/SNYK-RUBY-RACK-1061917

Gemfile

@@ -41,7 +41,7 @@ gem 'redis', '~> 4.2'
 gem 'jwt', '~> 2.2.2'
 gem 'oauth', '~> 0.5.1'
 
-gem 'doorkeeper', '~> 5.5.0'
+gem 'doorkeeper', '~> 5.6.6'
 gem 'repost', '~> 0.3.8'
 
 gem 'lodash-rails'

Gemfile.lock

@@ -139,7 +139,7 @@ GEM
     docile (1.4.0)
     domain_name (0.5.20190701)
       unf (>= 0.0.5, < 1.0.0)
-    doorkeeper (5.5.4)
+    doorkeeper (5.6.6)
       railties (>= 5)
     dotenv (2.8.1)
     dotenv-rails (2.8.1)
@@ -437,7 +437,7 @@ DEPENDENCIES
   byebug
   coffee-rails (~> 5.0, >= 5.0.0)
   coveralls
-  doorkeeper (~> 5.5.0)
+  doorkeeper (~> 5.6.6)
   dotenv-rails
   faraday
   font-awesome-sass (~> 6.4.0)
@@ -474,3 +474,6 @@ DEPENDENCIES
   tzinfo-data
   web-console (>= 4.2.1)
   webmock
+
+BUNDLED WITH
+   2.1.4