Constructor visibility was fixed for the Parallax serial controller. It wasn't possible to instantiate it via Spring before. If you're not using Parallax servo controller, or not using it with DIY Zoning or Spring, you don't need this release.
In servomaster-serial, the RxTx dependency was extracted from the Maven Central Repository. In servomaster-usb, Phidget QuadServo can now be used with Spring Framework (the DIY Zoning DZ3 branch requires this).