Update build process

[ivmartel]

• start using the git branching mode defined in a-successful-git-branching-model
• work on a develop branch
• release on the master branch
  -> change github branch settings to make develop the main one
• move yarn run build from the update script to the main travis file
• add a parameter to the update script to update trunk or stable demo
• stop building docs at each commit only do it for the master branch
• from travis, build trunk from the develop branch and stable from the master one

[ivmartel]

Also use lts node as travis build environment.

[ivmartel]

Made develop the default branch, I hope it does not break everything...