chore: update node version to 8

Author: Dimitar Todorov

.travis.yml

@@ -14,7 +14,7 @@ matrix:
       language: android
       os: linux
       jdk: oraclejdk8
-      before_install: nvm install stable
+      before_install: nvm install 8
       script:
         - cd src && npm i && npm run tsc && cd ../demo
         - travis_wait travis_retry tns build android
@@ -35,7 +35,7 @@ matrix:
         - TestVersion="latest"
       jdk: oraclejdk8
       before_install:
-        - nvm install stable
+        - nvm install 8
       before_script:
         - cd seed-tests && npm i
         - echo no | android create avd --force -n test -t android-21 -b armeabi-v7a