The AppVeyor tests run on 2.1 and 2.2, it seems, and they both fail.
"Too new Bundler" for those.
gem install bundler --no-document
ERROR: Error installing bundler:
The last version of bundler (>= 0) to support your Ruby & RubyGems was 1.17.3. Try installing it with gem install bundler -v 1.17.3
bundler requires Ruby version >= 2.3.0. The current ruby version is 2.1.0.
The AppVeyor tests run on 2.1 and 2.2, it seems, and they both fail.
"Too new Bundler" for those.