Updated ci build scripts (from rspec-dev)

[pirj]

rspec/rspec-dev#259

[pirj]

@JonRowe I've tried many options, and this one, the most straightforward, is the only one that worked.

Without unset RUBYOPT Rails is complaining that uninitialized constant Gem.
With --binstubs --standalone --path=../bundle and subsequent bin/rspec the binstub apparently ceases to find ../bundle:

# Gem::MissingSpecError:
#   Could not find 'sqlite3' (~> 1.4) among 30 total gem(s)
#   Checked in 'GEM_PATH=/home/runner/.gem/ruby/2.5.0:/opt/hostedtoolcache/Ruby/2.5.8/x64/lib/ruby/gems/2.5.0', execute `gem env` for more information

Would you agree for such a change?

• Yes, surely we're shaving some fractions of a second by using standalone, but is it worth such a complication?
• SPECS_HAVE_RUN_FILE is quite an odd flag to prevent duplicate runs, I can't think of a scenario where specs are started twice for the same repo
• [ -d ../$1 ] is useless - we do clone it, and if we don't - we don't run_spec_suite_for for this dir

To my huge surprise, the problem with uninitialized constant Gem can be reproduced with a small single commit comparing to the green 4-0-dev. That very commit that drops .travis.yml and barely does anything else.

[JonRowe]

I think its the ffi version, I had to fix rspec-core for main too, I've applied that patch to 4-0-dev and opened simplified PRs to see how they go

[pirj]

Awesome, thanks!
I'll close this series for now, we can address the removal of .travis.yml later.