build: add Node 8 tests (#459)

This PR was generated using Autosynth. 🌈

Synth log will be available here:
https://source.cloud.google.com/results/invocations/91408a5a-0866-4f1e-92b1-4f0e885b0e2e/targets

- [ ] To automatically regenerate this PR, check this box.

packages/google-cloud-videointelligence/synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/nodejs-video-intelligence.git",
-        "sha": "9c1de7477e7a050cc6da128d066ba07e81142970"
+        "sha": "74308b5e9930b4cdbbaad2fb87ec5d7b6537afbc"
       }
     },
     {