Stores the Setting.maximum(:updated_on) in a thread-local-variable

this gets rid of numerous redundant db-calls to check with every call to
a setting, if the settings have been updated. Checking this one and one
time only for a request should be sufficient.

let's see, if this works for the admin-specs to update settings.
Added the request-store, to make sure that thread-local variables are cleaned up after the request
replaced the pulled code with proper gem-reference

The railtie now automatically makes sure, that the middleware is used,
and the included gem better respects the copyright of the used solution.
forgot the removal of the middleware&the comment in application.rb
Changelog.
Merge pull request #490 from opf/fix/redundant_settings_max_updated_on

add a request-store and put the settings_max_updated_on into the request to avoid redundant calls