Grunt: Set testswarm/browserSets from JSON file.

File on jq03.jquery.com is already updated.

grunt.js

@@ -147,7 +147,7 @@ module.exports = function( grunt ) {
 			runMax: config.runMax,
 			"runNames[]": tests,
 			"runUrls[]": testUrls,
-			"browserSets[]": ["popular"]
+			"browserSets[]": config.browserSets
 		});
 	});