Remove explicit `require` calls, and use `require_dependency` when needed

See
http://guides.rubyonrails.org/autoloading_and_reloading_constants.html
for more info.
Signed-off-by: Rémy Coutable <remy@rymai.me>

app/controllers/omniauth_kerberos_spnego_controller.rb

-require 'omniauth/strategies/kerberos_spnego'
-
 class OmniauthKerberosSpnegoController < ApplicationController
   include KerberosSpnegoHelper
 

app/controllers/projects/wikis_controller.rb

-require 'project_wiki'
-
 class Projects::WikisController < Projects::ApplicationController
   before_action :authorize_read_wiki!
   before_action :authorize_create_wiki!, only: [:edit, :create, :history]

config/initializers/8_metrics.rb

@@ -142,9 +142,9 @@ if Gitlab::Metrics.enabled?
 
   # These are manually require'd so the classes are registered properly with
   # ActiveSupport.
-  require 'gitlab/metrics/subscribers/action_view'
-  require 'gitlab/metrics/subscribers/active_record'
-  require 'gitlab/metrics/subscribers/rails_cache'
+  require_dependency 'gitlab/metrics/subscribers/action_view'
+  require_dependency 'gitlab/metrics/subscribers/active_record'
+  require_dependency 'gitlab/metrics/subscribers/rails_cache'
 
   Gitlab::Application.configure do |config|
     config.middleware.use(Gitlab::Metrics::RackMiddleware)

config/initializers/omniauth.rb

@@ -25,11 +25,11 @@ end
 if Gitlab.config.omniauth.enabled
   provider_names = Gitlab.config.omniauth.providers.map(&:name)
   require 'omniauth-kerberos' if provider_names.include?('kerberos')
-  require 'omniauth/strategies/kerberos_spnego' if provider_names.include?('kerberos_spnego')
+  require_dependency 'omni_auth/strategies/kerberos_spnego' if provider_names.include?('kerberos_spnego')
 end
 
 module OmniAuth
   module Strategies
-    autoload :Bitbucket, Rails.root.join('lib', 'omniauth', 'strategies', 'bitbucket')
+    autoload :Bitbucket, Rails.root.join('lib', 'omni_auth', 'strategies', 'bitbucket')
   end
 end

lib/gitlab/ldap/user.rb

-require 'gitlab/o_auth/user'
-
 # LDAP extension for User model
 #
 # * Find or create user from omniauth.auth data

spec/lib/omniauth/strategies/kerberos_spnego_spec.rb

 require 'spec_helper'
-require 'omniauth/strategies/kerberos_spnego'
 
 describe OmniAuth::Strategies::KerberosSpnego do
   subject { described_class.new(:app) }

spec/requests/omniauth_kerberos_spnego_spec.rb

 require 'spec_helper'
-require 'omniauth/strategies/kerberos_spnego'
 
-describe 'OmniAuth Kerberos SPNEGO', lib: true do
+describe 'OmniAuth Kerberos SPNEGO' do
   let(:path) { '/users/auth/kerberos_spnego/negotiate' }
   let(:controller_class) { OmniauthKerberosSpnegoController }