Add nodejs versions 10 and 11 to CI, remove 6 and 9

.travis.yml

@@ -1,9 +1,9 @@
 language: node_js
 sudo: false
 node_js:
-  - "9"
+  - "11"
+  - "10"
   - "8"
-  - "6"
 install:
   - npm install
   - npm install codecov

appveyor.yml

@@ -5,9 +5,9 @@ init:
 # Test against these versions of Node.js.
 environment:
   matrix:
-    - nodejs_version: "9"
+    - nodejs_version: "11"
+    - nodejs_version: "10"
     - nodejs_version: "8"
-    - nodejs_version: "6"
 
 # Install scripts. (runs after repo cloning)
 install: