Remove dependency on admin_menu()

You have hard coded this into the submit handler of the admin form:

/**
 * Submit callback for the environment_indicator_settings form.
 */
function environment_indicator_settings_submit($form, $form_state) {
  admin_menu_flush_caches();
}

Which in turn causes a PHP fatal

( ! ) Fatal error: Call to undefined function admin_menu_flush_caches() in /var/www/d7/sites/all/modules/contrib/environment_indicator/environment_indicator.admin.inc on line 37

The fix is simple, just add a module_exists() wrapper around it. Raising as critical as this is clearly a mistake, and it actively prevents the module from working.