Ignore or properly resolve remote URLs

[paularmstrong]

Remote urls should be ignored or properly resolved. Right now, they are being searched as a path, ignoring the fact that they are on a fully-qualified domain:

RuntimeError: error evaluating function `data-uri`: ENOENT, no such file or directory '/var/www/https:/fonts.gstatic.com/s/sourcesanspro/v9/ODelI1aHBYDBqgeIAH2zlNHq-FFgoDNV3GTKpHwuvtI.woff' in /var/www/less/common/fonts.less on line 28, column 66:
27   font-weight: 400;
28   src: local('Source Sans Pro'), local('SourceSansPro-Regular'), url(https://fonts.gstatic.com/s/sourcesanspro/v9/ODelI1aHBYDBqgeIAH2zlNHq-FFgoDNV3GTKpHwuvtI.woff) format('woff');
29 }