Fix failing specs caused by $SAFE incompatibility on MRI 2.3

    ArgumentError:
      $SAFE=2 to 4 are obsolete

Author: yujinakayama

spec/support/helper_methods.rb

@@ -1,4 +1,6 @@
 module RSpecHelpers
+  SAFE_LEVEL_THAT_TRIGGERS_SECURITY_ERRORS = RUBY_VERSION >= '2.3' ? 1 : 3
+
   def relative_path(path)
     RSpec::Core::Metadata.relative_path(path)
   end
@@ -13,7 +15,7 @@ def ignoring_warnings
 
   def with_safe_set_to_level_that_triggers_security_errors
     Thread.new do
-      ignoring_warnings { $SAFE = 3 }
+      ignoring_warnings { $SAFE = SAFE_LEVEL_THAT_TRIGGERS_SECURITY_ERRORS }
       yield
     end.join