Add Time to whitelist of classes to load for YAML.safe_load

The update to ruby 2.3.1 pulls in a different version of the YAML / Psych code which doesn't allow the Time class to be loaded in `safe_load`, unlike the previous version. This change whitelists Time, and appears to solve the problem. Since `state.yaml` files are not loaded from the internet, and rarely hand-edited, then this seems like an okay thing to whitelist.