Use the actual available free memory instead of the max possible

src/com/sheepit/client/os/Mac.java

@@ -140,6 +140,11 @@ public class Mac extends OS {
 		return -1;
 	}
 	
+	@Override
+	public int getFreeMemory() {
+		return -1;
+	}
+	
 	@Override
 	public Process exec(List<String> command, Map<String, String> env) throws IOException {
 		List<String> actual_command = command;