Add properties to pom.xml

Without this the `mvn package` command complains. Here is a small sample, there are 15 similar errors.

```
/home/asa/horsekeep/HorseKeep/src/main/java/com/gmail/falistos/HorseKeep/HorseTeleportResponse.java:[3,7] enums are not supported in -source 1.3
(use -source 5 or higher to enable enums)
public enum HorseTeleportResponse {

/home/asa/horsekeep/HorseKeep/src/main/java/com/gmail/falistos/HorseKeep/commands/CommandAdminList.java:[26,6] generics are not supported in -source 1.3
(use -source 5 or higher to enable generics)
		List<String> horsesList = plugin.manager.getOwnedHorses(playerName);
```