Don't convert http urls to absolute path

The current version of LessBundle doesn't handle http urls in `url()` rules. For example:

    src: url('//themes.googleusercontent.com/static/fonts/opensans.woff');

gets replaced by:

    src: url(/themes.googleusercontent.com/static/fonts/opensans.woff);

When bundle optimisation is turned on. Note the "//themes" (incorrectly) gets replaced by "/themes".

This pull request fixes this issue, by explicitly checking if the url doesn't start with `//`, `http://`, or `https://`.