make regex case insensitive

[sarchila]

Not sure if this below is the intended behavior, but I've been running into issues with the case-sensitivity of the regex.

> var urlregexp = require('urlregexp')

> !!('google.com'.match(urlregexp)) // true
> !!('www.google.com'.match(urlregexp)) // true

> !!('Google.Com'.match(urlregexp)) // false
> !!('WWW.GOOGLE.COM'.match(urlregexp)) // false

This is a quick change that makes these all return true.