Fix failing specs caused by $SAFE incompatibility on MRI 2.3

[yujinakayama]

$SAFE level 2, 3, and 4 are no longer available on MRI 2.3.

https://bugs.ruby-lang.org/issues/5455#note-16

Failures:

  1) RSpec::Core::Formatters::HtmlSnippetExtractor falls back on a default message when it gets a security error
     Failure/Error: ignoring_warnings { $SAFE = 3 }

     ArgumentError:
       $SAFE=2 to 4 are obsolete
     # ./spec/support/helper_methods.rb:16:in `block (2 levels) in with_safe_set_to_level_that_triggers_security_errors'
     # ./spec/support/helper_methods.rb:9:in `ignoring_warnings'
     # ./spec/support/helper_methods.rb:16:in `block in with_safe_set_to_level_that_triggers_security_errors'

[JonRowe]

👍

[myronmarston]

Someone's on a roll!