Fix gulp-eslint errors

[sergioregueira]

The latest pull request #213 added some code that does not satisfy gulp-eslint requirements. I have changed a few things to solve the errors and unblock #217:

• Use a regular function instead of an arrow one.
• Wrap if block into brackets.
• Replace const with var.

[eugef]

Thanks for the fix, i think a better long-term solution would be to allow modern ES syntax that is supported by Node 10.

[sergioregueira]

That would require a transpiler (eg. babel) to keep the library compatible with Node.js 6.

If you agree, open a pull request and I will address it sometime.

[eugef]

Node.js org doesn't support versions below 10 https://nodejs.org/en/about/releases/
Neither should do we.