Updating changelog for 2.1.8 release.
Updating changelog for 2.1.8 release.

mcanini/java-driver,tolbertam/java-driver,mike-tr-adamson/java-driver,mebigfatguy/java-driver,adejanovski/java-driver,adejanovski/java-driver,tommystendahl/java-driver,fromanator/java-driver,kjniemi/java-driver,mike-tr-adamson/java-driver,DICL/cassandra-driver,DICL/cassandra-driver,mcanini/java-driver,tolbertam/java-driver,kjniemi/java-driver,fromanator/java-driver,kjniemi/java-driver,tolbertam/java-driver,tommystendahl/java-driver,DICL/cassandra-driver,mcanini/java-driver,mebigfatguy/java-driver,fromanator/java-driver,tommystendahl/java-driver,mike-tr-adamson/java-driver,adejanovski/java-driver,mebigfatguy/java-driver