Remove unneeded deploy.task blocks from deploy-passenger_nginx.rb

I believe these were copied from deploy-passenger.rb. I can't see any purpose for them in deploy-passenger_nginx.rb. I've tested creating a server, deploying, restarting Nginx, etc. and everything works without them.
Use update-alternatives to specify which version of Ruby to use.

This configures /usr/bin/ruby to run the version of Ruby installed by ruby-build.

This fixes a problem running passenger-status and passenger-memory-stats. Those commands appear to be hard-coded to use /usr/bin/ruby instead of using the Ruby in the path.