test: fix invalid config for jasmine-spec-reporter (#161)

Fixes the following warning:
    WARN: jasmine-spec-reporter 'displayStacktrace' option supports
    value ('none', 'raw', 'pretty'), default to 'none'
diff --git a/spec/helper.js b/spec/helper.js
index a4a5886..83669f3 100644
--- a/spec/helper.js
+++ b/spec/helper.js
@@ -26,6 +26,6 @@
     },
     summary: {
         displayDuration: true,
-        displayStacktrace: true
+        displayStacktrace: 'raw'
     }
 }));