use npm 6 (#572)

.travis.yml

@@ -39,7 +39,7 @@ addons:
 services:
   - redis-server
 before_install:
-  - npm install -g npm@5
+  - npm install -g npm@6
   - npm install -g greenkeeper-lockfile@1
   - ./bin/setup-mastodon-in-travis.sh
 before_script:

package-lock.json

@@ -4747,7 +4747,7 @@
             "colors": "~0.6.0",
             "js-yaml": "~2.1.0",
             "sax": "~0.6.0",
-            "whet.extend": "0.9.9"
+            "whet.extend": "^0.9.9"
           },
           "dependencies": {
             "whet.extend": {