update to node 6.9.11

https://nodejs.org/en/blog/vulnerability/july-2017-security-releases/

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 node_js:
-  - 6.9.1
+  - 6.11.1
 sudo: false
 addons:
   apt:

package.json

@@ -31,7 +31,7 @@
     "test-travis": "istanbul cover ./node_modules/mocha/bin/_mocha --report lcovonly -- -R spec && cat ./coverage/lcov.info | ./node_modules/coveralls/bin/coveralls.js && rm -rf ./coverage"
   },
   "engines": {
-    "node": "6.9.1"
+    "node": "6.11.1"
   },
   "dependencies": {
     "canvas": "^1.6.2",