Explictly require hangouts-chat in Gemfile

After the model `HangoutsChatService` was renamed
`Integrations::HangoutsChat`, and therefore the file renamed as
`hangsouts_chat.rb`,
`config/initializers/hangouts_chat_http_override.rb` began to define new
classes, rather than to monkeypatch the gem classes.

Requiring the gem this way allows us to continue to monkeypatch the gem.

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61957#note_586260689
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61957
https://gitlab.com/gitlab-org/gitlab/-/issues/201855

Gemfile

@@ -253,7 +253,7 @@ gem 'flowdock', '~> 0.7'
 gem 'slack-messenger', '~> 2.3.4'
 
 # Hangouts Chat integration
-gem 'hangouts-chat', '~> 0.0.5'
+gem 'hangouts-chat', '~> 0.0.5', require: 'hangouts_chat'
 
 # Asana integration
 gem 'asana', '~> 0.10.3'

app/models/integrations/hangouts_chat.rb

 # frozen_string_literal: true
 
-require 'hangouts_chat'
-
 module Integrations
   class HangoutsChat < BaseChatNotification
     include ActionView::Helpers::UrlHelper

spec/support/shared_examples/models/chat_integration_shared_examples.rb

@@ -53,7 +53,6 @@ RSpec.shared_examples "chat integration" do |integration_name|
       end
 
       it "calls #{integration_name} API" do
-        # binding.pry
         result = subject.execute(sample_data)
 
         expect(result).to be(true)