ci: test on Node.js 10 (#93)

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<!-- What changes are being made? (What feature/bug is being fixed here?) -->

**What**: CI

<!-- Why are these changes necessary? -->

**Why**: We should test against the last LTS branch, which is now 10.

<!-- How were these changes implemented? -->

**How**: Change from `8` to `10`.

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.travis.yml

@@ -5,7 +5,7 @@ cache:
     - ~/.npm
 notifications:
   email: false
-node_js: '8'
+node_js: '10'
 install: npm install
 script: npm run validate
 after_success: kcd-scripts travis-after-success