chore(ci): test Node.js 8, 10 and 11 (testing-library#148)

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

**What**:

All supported Node.js versions should be tested to catch any issues on newer LTS releases.

**Why**:

To ensure that there are no breaking builds on Node.js 6, 10 and 11.

**How**:

Updated the Travis CI config.

**Checklist**:

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- [ ] Documentation
- [ ] Tests
- [x] Ready to be merged <!-- In your opinion, is this ready to be merged as soon as it's reviewed? -->
- [ ] Added myself to contributors table <!-- this is optional, see the contributing guidelines for instructions -->

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Author: DanielRuf

.travis.yml

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

package.json

@@ -21,7 +21,7 @@
   "author": "Kent C. Dodds <[email protected]> (http://kentcdodds.com/)",
   "license": "MIT",
   "engines": {
-    "node": ">=6"
+    "node": ">=8"
   },
   "scripts": {
     "add-contributor": "kcd-scripts contributors add",