SmartBOB-USB plug-in updated to version 0.51.252 probing fixes, support PMDX-414

[Steve Stallings]

The Mach4Hobby plugin for the SmartBOB-USB family of controllers has been updated to version 0.51.252,
released 19 April 2017.  This plug-in requires Mach4 builds 2872 or higher. It has been tested with builds
2914 and 2233, and should work with later builds when released.

Full details of this release are here: http://www.pmdx.com/Downloads_/ReleaseNotes_Ver_0_51_252.html

Download the plug-in here:  http://www.pmdx.com/PMDX-SmartBOB-USB-Plug-In

Major changes for this release (see the release notes for a complete list of changes):

(1) Fixed several bugs in probing support.

(2) Clarified error messages for lathe threading.

(3) Added support for PMDX-414.