Fixed beyond compare to use the blocking executable

[benPearce1]

fixes https://github.com/droyad/Assent/issues/41

Only tested on MacOS, not able to test on Windows.

Reference docs https://www.scootersoftware.com/v4help/index.html?command_line_reference.html

[adamchester]

friendly bump - any plans to move on this one?

[droyad]

Ah yeh. I'll catch up with you @benPearce1 next week. I have concerns how this will behave on Windows.

[rhysparry]

I hit this one recently too. Issue was on a Mac. If there are concerns about the Windows change is it worth splitting off the fix for Mac?