chore(CI): fix node.js version on Travis and AppVeyor (puppeteer#2180)

AppVeyour was configured to use the latest versions of Node for major releases. To make builds more reproducible I've changed both Travis and AppVeyor to use the same fixed versions of Node 6 and Node 7 that AppVeyor is using at the moment.

.appveyor.yml

@@ -1,17 +1,17 @@
-environment:
-  matrix:
-    - nodejs_version: "6"
-    - nodejs_version: "7"
-
-build: off
-
-install:
-  - ps: Install-Product node $env:nodejs_version
-  - npm install
-  - if "%nodejs_version%" == "7" (
-      npm run lint &&
-      npm run coverage  &&
-      npm run test-doclint
-    ) else (
-      npm run unit-node6
-    )
+environment:
+  matrix:
+    - nodejs_version: "6.12.3"
+    - nodejs_version: "7.10.1"
+
+build: off
+
+install:
+  - ps: Install-Product node $env:nodejs_version
+  - npm install
+  - if "%nodejs_version%" == "7.10.1" (
+      npm run lint &&
+      npm run coverage  &&
+      npm run test-doclint
+    ) else (
+      npm run unit-node6
+    )

.travis.yml

@@ -19,9 +19,9 @@ script:
   - 'if [ "$NODE6" = "true" ]; then npm run unit-node6; fi'
 jobs:
   include:
-    - node_js: "7.6.0"
+    - node_js: "7.10.1"
       env: NODE7=true
-    - node_js: "6.4.0"
+    - node_js: "6.12.3"
       env: NODE6=true
 before_deploy: "npm run apply-next-version"
 deploy: