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/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

@@ -120,9 +120,9 @@ 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

@@ -20,15 +20,12 @@ OmniAuth.config.before_request_phase do |env|
 end
 
 if Gitlab.config.omniauth.enabled
-  Gitlab.config.omniauth.providers.each do |provider|
-    if provider['name'] == 'kerberos'
-      require 'omniauth-kerberos'
-    end
-  end
+  provider_names = Gitlab.config.omniauth.providers.map(&:name)
+  require 'omniauth-kerberos' if provider_names.include?('kerberos')
 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