Change default Etherpad branch to 'develop'

With recent changes to NodeJS and npm packages, Etherpad no longer wants
to install from '1.4.0' version, and even '1.4.1' version. For the time
being, it will be installed from 'develop' branch until next release.
Merge pull request #2 from drybjed/switch-version

Change default Etherpad branch to 'develop'