Don't rewrite network path references in CSS URLs.

Fixes regression caused in 1.3.4.1 which caused network path reference URLs (i.e. //img.domain.com/path/font.eot) used in CSS `url()`'s' to be stripped of their leading slashes causing them to become relative URL paths.

Closes meteor/meteor#7416

packages/minifier-css/minifier.js

@@ -136,8 +136,9 @@ var rewriteRules = function (rules, mergedCssPath) {
 
 
         // We don't rewrite URLs starting with a protocol definition such as
-        // http, https, or data.
-        if (resource.protocol !== null) {
+        // http, https, or data, or those with network-path references
+        // i.e. //img.domain.com/cat.gif
+        if (resource.protocol !== null || resource.href.startsWith('//')) {
           continue;
         }