Updated readme to reflect new composer requirements.
Updated readme to reflect new composer requirements.

digideskio/libphonenumber-for-php,gencer/libphonenumber-for-php,Xephi/libphonenumber-for-php,BenMorel/libphonenumber-for-php,giggsey/libphonenumber-for-php,lukebeer/libphonenumber-for-php,sudiptpa/libphonenumber-for-php