make import use shared relative path function

Author: lukeapage

lib/less/tree/import.js

@@ -68,7 +68,7 @@ tree.Import.prototype = {
         if (this.rootpath && !(path instanceof tree.URL)) {
             var pathValue = path.value;
             // Add the base path if the import is relative
-            if (pathValue && !/^(?:[a-z\-]+:|\/)/.test(pathValue)) {
+            if (pathValue && env.isPathRelative(pathValue)) {
                 path.value =  this.rootpath + pathValue;
             }
         }