Support redis-rb v4

Also:
 - Stub #connection method on MockRedis instances

sidekiq-scheduler.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |s|
   s.files       = Dir['{lib,web}/**/*'] + %w[MIT-LICENSE Rakefile README.md]
 
   s.add_dependency 'sidekiq',         '>= 3'
-  s.add_dependency 'redis',           '~> 3'
+  s.add_dependency 'redis',           '>= 3', '< 5'
   s.add_dependency 'rufus-scheduler', '~> 3.2'
   s.add_dependency 'tilt',            '>= 1.4.0'
 

spec/support/initialization/redis.rb

@@ -4,13 +4,13 @@
 RSpec.configure do |config|
   config.before do
     redis = MockRedis.new
-    client = Object.new
 
-    client.define_singleton_method(:id) do
-      "redis://127.0.0.1:1234/0"
-    end
+    connection = {
+      location: '127.0.0.1:1234',
+      db: '0'
+    }
 
-    allow(redis).to receive(:client).and_return(client)
+    redis.define_singleton_method(:connection) { connection }
 
     allow(Sidekiq::RedisConnection).to receive(:create).and_return(ConnectionPool.new({}) {
       redis