Updated read me with info for new objection_require_sel 

I've updated the readme file so that it contains a description about new objection_requires_sel. 

I've also updated podspec file and changed the way it looks for a given version Instead of specifically defining tag it now takes previously defined version - one less parameter to worry about ;)

Could you release a new pod version in the main pods repository? I'm really eager to use the new feature! 

Thanks!