Use strings prefixed by module name as requirement keys in hook_requirements()

In its implementation of hook_requirements(), geshifilter uses integers as requirement keys. Since the new status page in Drupal 8.3 uses switch statements to render some requirement keys differently and switch statement comparisons are loosely typed, this leads to the following error on the status page:

Warning: Invalid argument supplied for foreach() in Drupal\system\Element\StatusReportPage::preRenderGeneralInfo() (line 43 of core/modules/system/src/Element/StatusReportPage.php). array(19)

The hook_requirements() documentation says that requirement keys are arbitrary but must be unique. This is technically more like a core bug, since the current implementation fulfils that, as long as no other module uses integer keys for its requirements. However, the documentation also says, that it is suggested to use the module short name as a prefix. Also, it is easy to imagine a case, where the current implementation might return non-unique keys, if another module uses a similar implementation. So it is probably a good idea to adhere to the suggested best practice and use string keys prefixed by module name.