Temporarily disable Node 8 CI to unblock release

CI is broken for Node 8. We're dealing with this in #1870 so I'm removing this for now, so we can the patch for #1836 shipped.

.travis.yml

@@ -40,9 +40,6 @@ jobs:
     - node_js: 10
       # Avoid running lint and prettier again.
       script: npm run --silent unit && npm run mocha
-    - node_js: 8
-      # Avoid running lint and prettier again.
-      script: npm run --silent unit && npm run mocha
     - stage: release
       node_js: lts/*
       env: semantic-release