Updated capistrano from 2.15.5 to 3.0.1

This is a major update which breaks backward compatibility in the Capistrano DSL.

WIP: adapt Capistrano configuration to the new version, following the official guide and a related blog post from Semaphore CI:

http://www.capistranorb.com/documentation/upgrading/
https://semaphoreapp.com/blog/2013/11/26/capistrano-3-upgrade-guide.html
Updated capistrano configuration to version 3 of the gem.