Add node 10 to travis and appveyor matrix

Also run integration tests with node 8 instead of 6 in appveyor

Author: tolbertam

.travis.yml

@@ -1,5 +1,6 @@
 language: node_js
 node_js:
+  - "10"
   - "8"
   - "6"
   - "4"

ci/appveyor.yml

@@ -2,10 +2,11 @@ environment:
   appveyor_build_worker_cloud: gce
   ci_type: ci_unit
   matrix:
-    - nodejs_version: 6
+    - nodejs_version: 8
       TEST_CASSANDRA_VERSION: 3.11.2
       ci_type: ci
-    - nodejs_version: 8
+    - nodejs_version: 10
+    - nodejs_version: 6 
     - nodejs_version: 4
 image: Visual Studio 2017
 platform: