Don't use arrow functions and const to work on IE11

[guillaumewuip]

The library cannot be used out-of-the-box on IE11 because IE6 is not supported there.

This PR simply convert the two arrow functions into classic functions and replace const by var

[Richienb]

I feel like this won't be needed since this PR needs to be compiled either way to use module.exports.

[niftylettuce]

This issue is fixed in my maintained and modern version of this package at https://github.com/niftylettuce/url-regex-safe. You should be able to switch from url-regex to url-regex-safe now. See the updated list of options as I added some new ones, and changed a few defaults to more sensible ones (since not everyone is parsing Markdown for instance).