After adding the gem "webmock" I have "uninitialized constant Syck (NameError)"

I've added gem "webmock" and started having an error:

$ rspec
/home/me/.gem/ruby/2.3.1/gems/safe_yaml-1.0.1/lib/safe_yaml/syck_node_monkeypatch.rb:42:in `<top (required)>': uninitialized constant Syck (NameError)

I've tried includig this:

 module SafeYAML
  YAML_ENGINE = defined?(YAML::ENGINE) ? YAML::ENGINE.yamler : (defined?(Psych) && YAML == Psych ? "psych" : "syck")
end

into different initialization files such as application.rb and initializers/my_init.rb, but it didn't help to fix the error. How can I fix it?