Add Node 10 to Appveyor config. Remove Node 6.

Author: iansu

appveyor.yml

@@ -2,21 +2,21 @@ image: Visual Studio 2017
 
 environment:
   matrix:
-    - nodejs_version: 8
+    - nodejs_version: 10
       test_suite: "simple"
-    - nodejs_version: 8
+    - nodejs_version: 10
       test_suite: "installs"
-    - nodejs_version: 8
+    - nodejs_version: 10
       test_suite: "kitchensink"
     - nodejs_version: 8
       test_suite: "kitchensink-eject"
     - nodejs_version: 8
       test_suite: "monorepos"
-    - nodejs_version: 6
+    - nodejs_version: 8
       test_suite: "simple"
-    - nodejs_version: 6
+    - nodejs_version: 8
       test_suite: "installs"
-    - nodejs_version: 6
+    - nodejs_version: 8
       test_suite: "kitchensink"
     - nodejs_version: 6
       test_suite: "kitchensink-eject"