Update build-unix.md
Update build-unix.md

specify boost >= 1.55
update centos to build newer libboost

Former-commit-id: 914c79db7b7c2349af7464bc858010dbe43c71ef
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin