Puma CI fails when using Bundler 2

[eregon]

See https://github.com/puma/puma/runs/553627450

/home/runner/.rubies/truffleruby-head/lib/mri/bundler/lockfile_parser.rb:108:in `warn_for_outdated_bundler_version': You must use Bundler 2 or greater with this lockfile. (Bundler::LockfileError)
	from /home/runner/.rubies/truffleruby-head/lib/mri/bundler/lockfile_parser.rb:95:in `initialize'
	from /home/runner/.rubies/truffleruby-head/lib/mri/bundler/definition.rb:83:in `new'
	from /home/runner/.rubies/truffleruby-head/lib/mri/bundler/definition.rb:83:in `initialize'
	from /home/runner/.rubies/truffleruby-head/lib/mri/bundler/dsl.rb:234:in `new'
	from /home/runner/.rubies/truffleruby-head/lib/mri/bundler/dsl.rb:234:in `to_definition'
	from /home/runner/.rubies/truffleruby-head/lib/mri/bundler/dsl.rb:13:in `evaluate'
	from /home/runner/.rubies/truffleruby-head/lib/mri/bundler/definition.rb:34:in `build'
	from /home/runner/.rubies/truffleruby-head/lib/mri/bundler.rb:135:in `definition'
	from /home/runner/.rubies/truffleruby-head/lib/mri/bundler.rb:101:in `setup'
	from /home/runner/.rubies/truffleruby-head/lib/mri/bundler/setup.rb:20:in `<top (required)>'
	from bin/puma:1:in `require'
	from bin/puma:1:in `<main>'

Which looks like the shipped bundler (which is Bundler 1) gets required instead of Bundler 2, when the bundle install before was done with Bundler 2.

It most likely happens since ruby/setup-ruby installs Ruby 2 by default if there is no Gemfile.lock:
ruby/setup-ruby#38 (comment)

cc @nateberkopec

[eregon]

I cannot reproduce the failure locally (with truffleruby-jvm).
For now let's use Bundler 1 in Puma's CI, there are other issues notably on Ruby 2.3 with Bundler 2:
puma/puma#2214

[bjfish]

This may be related to #1987

[eregon]

FWIW, the failure on MRI 2.3 with Bundler 2 is most likely due to rubygems/rubygems#3570 (comment)

[eregon]

TruffleRuby now works fine with Bundler 2 when running in Puma's CI: https://github.com/eregon/puma/runs/647762836

#1987 and #1956 fixed this.

I'll make a PR to Puma to use the latest Bundler once the Ruby 2.3 bug is resolved (ruby/setup-ruby#51).