try bumping appveyor/travis npm version

.travis.yml

@@ -4,6 +4,8 @@ node_js:
 addons:
     code_climate:
         repo_token: 7454b1a666015e244c384d19f48c34e35d1ae58c3aa428ec542f10bbcb848358
+before_install:
+    - npm i -g npm@latest
 script:
     - npm run lint
     - npm run test

appveyor.yml

@@ -7,7 +7,7 @@ install:
   # Get the latest stable version of Node.js or io.js
   - ps: Install-Product node $env:nodejs_version
   # install modules
-  - npm install -g npm@5.7
+  - npm install -g npm@latest
   - npm install
 
 # Post-install test scripts.