Unset no-rubygems option for rspec-rails sub-job

Rails do use Gem in rails/railties/lib/rails/ruby_version_check.rb

    if Gem::Version.new(RUBY_VERSION) < Gem::Version.new("2.5.0")

and obviously since RubyGems are loaded by default by recent rubies, it
doesn't care to explicitly require it. That results in:

    /home/runner/work/rspec-expectations/rspec-rails/bin/rspec

    An error occurred while loading spec_helper.
    Failure/Error: require 'rails/all'

    NameError:
      uninitialized constant Gem
    # /home/runner/work/rspec-expectations/bundle/ruby/2.5.0/gems/railties-6.0.3.4/lib/rails/ruby_version_check.rb:3:in `<top (required)>'
    # /home/runner/work/rspec-expectations/bundle/ruby/2.5.0/gems/railties-6.0.3.4/lib/rails.rb:3:in `require'

Author: pirj

ci/script/functions.sh

@@ -82,6 +82,9 @@ function run_spec_suite_for {
       unset BUNDLE_GEMFILE
       bundle_install_flags="--binstubs --standalone --without documentation --path ../bundle"
       travis_retry eval "(unset RUBYOPT; exec bundle install $bundle_install_flags)"
+      if [ $1 == rspec-rails ]; then
+        unset RUBYOPT
+      fi;
       run_specs_and_record_done
       popd
     else