Fix overflow in os.epoch

Closes #611

src/main/java/dan200/computercraft/core/apis/OSAPI.java

@@ -412,7 +412,7 @@ public class OSAPI implements ILuaAPI
                 // Get in-game epoch
                 synchronized( m_alarms )
                 {
-                    return m_day * 86400000 + (int) (m_time * 3600000.0f);
+                    return m_day * 86400000L + (long) (m_time * 3600000.0);
                 }
             default:
                 throw new LuaException( "Unsupported operation" );