We need to rebuild the binary whenever the plugin has changed.
Previously we were not doing this if the binary already existed. This MR
adds an override parameter to the build() function which will force a
rebuild even if the binary already exists.
The README has been updated to note that this parameter should be used
for Packer/Lazy, and when someone is troubleshooting. It won't be passed
in to the build function during the normal setup call.
Should address #55
ac2118ceb6