[README] fix the required PHP version
[README] fix the required PHP version

Code requirements have changed, see issue #390
Riduidel/rss-bridge,kranack/rss-bridge,RSS-Bridge/rss-bridge,ORelio/rss-bridge,simonbru/rss-bridge,griffaurel/rss-bridge,sebsauvage/rss-bridge,simonbru/rss-bridge,LogMANOriginal/rss-bridge,ORelio/rss-bridge,Riduidel/rss-bridge,GregThib/rss-bridge,LogMANOriginal/rss-bridge,Riduidel/rss-bridge,RSS-Bridge/rss-bridge,kranack/rss-bridge,teromene/rss-bridge,teromene/rss-bridge,polo2ro/rss-bridge,kranack/rss-bridge,Frenzie/rss-bridge,mro/rss-bridge,mro/rss-bridge,RSS-Bridge/rss-bridge,ORelio/rss-bridge,Frenzie/rss-bridge,kranack/rss-bridge