chore: udpate node version in travis builds

Author: Dimitar Todorov

.travis.yml

@@ -6,7 +6,7 @@ matrix:
     - stage: "Lint"
       language: node_js
       os: linux
-      node_js: "6"
+      node_js: "8"
       script: cd src && npm run ci.tslint && cd ../demo && npm run ci.tslint
     - stage: "WebPack and Build"
       os: osx
@@ -16,7 +16,7 @@ matrix:
         - Webpack="true"
       osx_image: xcode9.2
       language: node_js
-      node_js: "6"
+      node_js: "8"
       jdk: oraclejdk8
       script: cd demo && npm run build.plugin && npm i && tns build ios --bundle --env.uglify
     - os: linux
@@ -26,7 +26,7 @@ matrix:
         - AndroidSDK="26"
         - Webpack="true"
       jdk: oraclejdk8
-      before_install: nvm install 6.10.3
+      before_install: nvm install 8.11.4
       script: >
         echo { \"project_info\": { \"project_number\": \"424050927453\", \"firebase_url\": \"https://pptest-53916.firebaseio.com\", \"project_id\": \"pptest-53916\", \"storage_bucket\": \"pptest-53916.appspot.com\" }, \"client\": [ { \"client_info\": { \"mobilesdk_app_id\": \"1:424050927453:android:21d2dd8987e1e3d1\", \"android_client_info\": { \"package_name\": \"org.nativescript.ppTest\" } }, \"oauth_client\": [ { \"client_id\": \"424050927453-o7gv6bcmknujjoaaoa5r4d452cutb9jf.apps.googleusercontent.com\", \"client_type\": 3 } ], \"api_key\": [ { \"current_key\": \"AIzaSyDr3fqH7Z8-vhBNpAges_KFTA7oOoylGvU\" } ], \"services\": { \"analytics_service\": { \"status\": 1 }, \"appinvite_service\": { \"status\": 1, \"other_platform_oauth_client\": [] }, \"ads_service\": { \"status\": 2 } } } ], \"configuration_version\": \"1\"} > demo/app/App_Resources/Android/google-services.json &&
         cd demo && npm run build.plugin && npm i && tns build android --bundle --env.uglify
@@ -36,7 +36,7 @@ matrix:
         - AndroidSDK="26"
       os: linux
       jdk: oraclejdk8
-      before_install: nvm install 8.9.4
+      before_install: nvm install 8.11.4
       script: >
         echo { \"project_info\": { \"project_number\": \"424050927453\", \"firebase_url\": \"https://pptest-53916.firebaseio.com\", \"project_id\": \"pptest-53916\", \"storage_bucket\": \"pptest-53916.appspot.com\" }, \"client\": [ { \"client_info\": { \"mobilesdk_app_id\": \"1:424050927453:android:21d2dd8987e1e3d1\", \"android_client_info\": { \"package_name\": \"org.nativescript.ppTest\" } }, \"oauth_client\": [ { \"client_id\": \"424050927453-o7gv6bcmknujjoaaoa5r4d452cutb9jf.apps.googleusercontent.com\", \"client_type\": 3 } ], \"api_key\": [ { \"current_key\": \"AIzaSyDr3fqH7Z8-vhBNpAges_KFTA7oOoylGvU\" } ], \"services\": { \"analytics_service\": { \"status\": 1 }, \"appinvite_service\": { \"status\": 1, \"other_platform_oauth_client\": [] }, \"ads_service\": { \"status\": 2 } } } ], \"configuration_version\": \"1\"} > demo/app/App_Resources/Android/google-services.json &&
         cd demo && npm run ci.android.build
@@ -46,7 +46,7 @@ matrix:
         - Xcode="9.2"
       osx_image: xcode9.2
       language: node_js
-      node_js: "6"
+      node_js: "8"
       jdk: oraclejdk8
       script: cd demo && npm run ci.ios.build