Fix a permission problem by using the hashCode method from Objects.

Should fix:
20:14:24.740 [main] ERROR o.t.module.sandbox.ModuleClassLoader - Denied access to class (not allowed with this module's permissions): com.google.common.hash.HashCode
20:14:24.745 [main] ERROR o.t.module.sandbox.ModuleClassLoader - Denied access to class (not allowed with this module's permissions): com.google.common.hash.HashCode$BytesHashCode

in log from:
http://forum.terasology.org/threads/terasology-server-crash-issue-with-log-file.1166/