Fix memory leak with ruby/require/autoload_paths

In ruby versions less than 2.5.0, it was found that if you have
Pathname objects in the $LOAD_PATH instead of regular strings, it messes
with ruby's internals and causes a memory leak.

In most ruby applications, this wouldn't be much of an issue, but there
are a significant number of deferred require statements that we do in
manageiq that cause this to leak overtime.  Further more, this seems to
be an issue that presents itself even if the file has been loaded
previously (mostly a no-op for require).

Replication of this issue can be done using a simple ruby script:

    require 'pathname'
    require 'fileutils'

    puts Process.pid

    Dir.mkdir("foo") unless Dir.exists?("foo")
    $LOAD_PATH.unshift(Pathname.new("foo"))

    FileUtils.touch("empty.rb")
    1500.times { 1500.times { require "empty" }; print "."; GC.start; }

By simply running the application with the Pathnames converted to
strings, this should be a proper and low cost workaround for us until it
is patched in ruby or we update manageiq to >= ruby 2.5.0.

lib/manageiq/ui/classic/engine.rb

@@ -31,8 +31,8 @@ module ManageIQ
   module UI
     module Classic
       class Engine < ::Rails::Engine
-        config.autoload_paths << root.join('app', 'controllers', 'mixins')
-        config.autoload_paths << root.join('lib')
+        config.autoload_paths << root.join('app', 'controllers', 'mixins').to_s
+        config.autoload_paths << root.join('lib').to_s
         if Rails.env.production? || Rails.env.test?
           require 'uglifier'
           config.assets.js_compressor = Uglifier.new(