Pin puppeteer dependency to v1.5.0

The latest release of puppeteer (v1.6.0) is causing errors in our
smoke tests. We're pinning puppeteer to v1.5.0 until we can track
down the cause of the issue.

Once we have n-test working with puppeteer v1.6.0, we can get back
to pinning the puppeteer dependency more loosely i.e. v1.x.

 🐿 v2.10.0

package.json

@@ -29,7 +29,7 @@
         "directly": "^2.0.6",
         "inquirer": "^5.0.1",
         "node-fetch": "^2.1.1",
-        "puppeteer": "^1.5.0",
+        "puppeteer": "1.5.0",
         "wdio-sauce-service": "^0.4.8",
         "webdriverio": "^4.11.0"
     },