Use regular expression to replace all occurrences

lib/carto/tree/value.js

@@ -33,7 +33,7 @@ tree.Value.prototype = {
       var val = this.ev(env);
       var v = val.toString();
       if(val.is === "color" || val.is === 'uri' || val.is === 'string' || val.is === 'keyword') {
-        v = "'" + v.replace('&', '&') + "'";
+        v = "'" + v.replace(/&/g, '&') + "'";
       } else if (Array.isArray(this.value) && this.value.length > 1) {
         // This covers something like `line-dasharray: 5, 10;`
         // where the return _value has more than one element.