Fix to continue support es5

[archmoj]

@rreusser FYI - on plotly.js repo we had problem upgrading because of those es6 keywords.

[rreusser]

Thanks for the PR, @archmoj! It looks good and passes tests, so I've published this as v1.5.1. I tried adding parserOptions to the standard config in package.json, but didn't immediately succeed in enforcing ES5 as part of the tests in order to prevent this from happening in the future. I'm sure it's not hard, but I'd have to dig a bit deeper to figure out the right trick to get standard to accept this option.