global variable `$thor_runner' not initialized

I'm getting warning when running my program (which uses Thor) with `$VERBOSE = true`:

    /home/wolf/.rvm/gems/ruby-2.5.3@sm-toolkit/gems/thor-0.20.0/lib/thor.rb:396: warning: global variable `$thor_runner' not initialized

simple repro:

```
#!/usr/bin/ruby

$VERBOSE = true

require 'thor'

class Test < Thor
end

Test.start(ARGV)
```