test: fix missing stack traces in jasmine output (#1012)

`true` is not a valid value for the `displayStacktrace` option of
jasmine-spec-reporter.

spec/helper.js

@@ -26,6 +26,6 @@ jasmine.getEnv().addReporter(new SpecReporter({
     },
     summary: {
         displayDuration: true,
-        displayStacktrace: true
+        displayStacktrace: 'raw'
     }
 }));