Add jackson JDK8 serializers to properly deserialize dropwizard-cassandra configuration

[chrislbs]

I was trying to integrate authentication with cassandra and noticed that the version of dropwizard-cassandra being used utilized java.util.Optional rather than Guava's Optional. Because the version of dropwizard being used in the project doesn't automatically add these serializers to the object mapper, we need to explicitly add them during bootstrap.