Run CI using the default version of Bundler [changelog skip]

[eregon]

Description

Run CI using the default version of Bundler

• Which is Bundler on Ruby 2.2 and Bundler 2 on Ruby 2.3+.

This used to fail on TruffleRuby (fixed in oracle/truffleruby#1984) and on Ruby 2.3 (due to ruby/setup-ruby#51, now fixed).

Do not set the --path for bundle install

• This is buggy with Ruby 2.3 + Bundler 2.1.4, see cannot load such file -- bundler/gem_tasks with Ruby 2.3 and Bundler 2 ruby/setup-ruby#51

cc @MSP-Greg, I'm not sure why the --path was set but it doesn't seem needed.

[nateberkopec]

@MSP-Greg all you on this one

[MSP-Greg]

@nateberkopec

Thanks.

@eregon and I spent some of today working on changes to the actions used here and elsewhere, and both of us ran Puma CI in our forks to test them.

There may be some changes tomorrow. I'm tired of thinking about Actions workflows right now...

[dentarg]

Re: --path, it was added in #2126, not sure in reaction to what.

In the past, the old build system, Travis, used --path: #1893 (comment)

[eregon]

Right, --path is often used for caching.
Bundler caching is coming in ruby/setup-ruby#56 and setting an explicit value conflicts with that (i.e., does not use the cache), so let's remove it.

[MSP-Greg]

I think --path is being discouraged in favor of bundle config, so probably best to remove. Caching can be done in another PR.

LGTM