php 7.2 compatibility

Hi

Since we start using your great plugin on old server with php 7.2 there is one small suggestion.

add 3 lines in xfgmc_get_first_feed_id function like below

if (version_compare(PHP_VERSION, '7.3.0') >= 0) {
			return array_key_first($xfgmc_settings_arr);
        } else {
        	reset($xfgmc_settings_arr); 
			return key($xfgmc_settings_arr);
        }

So it will be working as usual without php fatal errors.