Ensure the same npm version is used for testing

sindresorhus · sindresorhus · commit 7ad16090b09d · 2018-06-25T00:09:00.000+07:00
diff --git a/.travis.yml b/.travis.yml
@@ -3,3 +3,5 @@ node_js:
   - '10'
   - '8'
   - '6'
+before_install:
+  - npm install --global npm@latest
