Release 0.2.1 is a port of release 0.2.0. It will run on the newest Rubies (Ruby 2.0 - 2.4 and beyond). 0.2.0 will only run on Rubies up to 2.2.
If you have a 2.x Ruby, you should run this release. If you have a 1.x Ruby, you should run the 0.2.0 release.
Unless noted otherwise, future Virtual Wiring Grapher releases will run on only 2.x Rubies.
To learn the version of Ruby you are running, type "ruby -v" at your command prompt.