Make JS use API v3 because v4 is not frozen yet

lib/gitlab/gon_helper.rb

 module Gitlab
   module GonHelper
     def add_gon_variables
-      gon.api_version            = API::API.version
+      gon.api_version            = 'v3' # v4 Is not officially released yet, therefore can't be considered as "frozen"
       gon.default_avatar_url     = URI.join(Gitlab.config.gitlab.url, ActionController::Base.helpers.image_path('no_avatar.png')).to_s
       gon.max_file_size          = current_application_settings.max_attachment_size
       gon.relative_url_root      = Gitlab.config.gitlab.relative_url_root