feat(runner): return a promise from runner.runOnce

In some cases knowing when the runner has finished is a requirement (e.g. an async grunt task).

lib/runner.js

@@ -304,12 +304,12 @@ var runJasmineTests = function() {
  * Run Protractor once.
  */
 var runOnce = function() {
-  setUpSelenium().then(function() {
+  return setUpSelenium().then(function() {
     // cleanUp must be registered directly onto runJasmineTests, not onto
     // the chained promise, so that cleanUp is still called in case of a
     // timeout error. Timeout errors need to clear the control flow, which
     // would mess up chaining promises.
-    runJasmineTests().then(cleanUp);
+    return runJasmineTests().then(cleanUp);
   });
 };