Deprecate safe_level of ERB.new in Ruby 2.6

lib/rspec/core/configuration_options.rb

@@ -168,7 +168,13 @@ def args_from_options_file(path)
       end
 
       def options_file_as_erb_string(path)
-        ERB.new(File.read(path), nil, '-').result(binding)
+        # FIXME: Consider removing the following conditional branch after Ruby 2.6 is released.
+        #        https://github.com/ruby/ruby/commit/cc777d09f44fa909a336ba14f3aa802ffe16e010
+        if RUBY_VERSION >= '2.6'
+          ERB.new(File.read(path), :trim_mode => '-').result(binding)
+        else
+          ERB.new(File.read(path), nil, '-').result(binding)
+        end
       end
 
       def custom_options_file