[#1374] Travis: utilise latest node version for PR docs deployment (#…

…1381)

The deployment of PR docs are used with an outdated node version, 
which causes the building of Markbind to fail.

Setting the node version to the latest version will fix the issue, 
as Markbind requires node version >= v10.13.0 installed.

Let's specify the node version for the deployment to be `lts/*`, 
which will ensure that the Markbind runs without errors.

.travis.yml

@@ -21,6 +21,8 @@ matrix:
       language: java
       os: linux
       jdk: openjdk8
+      env:
+        - NODE_VERSION="lts/*"
       before_install:
         - sudo apt-get update
         - sudo apt-get --only-upgrade install openjdk-8-jdk
@@ -77,6 +79,8 @@ after_success:
   - |
     if [ "$TRAVIS_JOB_NAME" = "Ubuntu 14.04 JDK 8" ];
     then
+      nvm install $NODE_VERSION
+      nvm use $NODE_VERSION
       git fetch --all && git config --global user.email "-" && git config --global user.name "-" && ./gradlew run -Dargs="--since d1"
       npm install -g surge
       npm install -g markbind-cli