fix: updates URL_REGEX to handle minified single css propery
Patch fixes this:

Works:
.a-background-image {background-image:url(/path/to/image.jpg);}
Don't work
.a-background-image {background-image:url(/path/to/image.jpg)}

Issue occured with https://github.com/GoalSmashers/clean-css
Merge pull request #10 from ef-ctx/master

fix: updates URL_REGEX to handle minified single css propery