Add node 6 support and allow installation of node version via nvm

[philipgough]

ping @odra

Install node 6 latest by default and change perms on nvm dir to allow users to run nvm install in Jenkinsfile

[odra]

👀

[odra]

nvm working with the following commands:

node('android') {
    stage('nvm') {
        sh 'node --version'
        sh '. /opt/nvm/nvm.sh && nvm install 4.4.3 && nvm alias default 4.4.3'
        sh '. /opt/nvm/nvm.sh && node --version'
    }
}
``'

[odra]

@philipgough let's just change the default node version to "6.9.1" then feel free to merge it 👍