update rbenv docs/example to work on Ubuntu/Debian

The included example here was failing on Ubuntu (tested on 14.04). Libssl-dev is required for some ruby versions, and 
the ruby-build docs recommend additional dependencies, which I think we should follow for at least the canonical example 
given in the docs.

According to the ruby-build wiki, the recommended dependencies for `rbenv install` (ruby-build) on ubuntu/debian are:
`autoconf bison build-essential libssl-dev libyaml-dev libreadline6-dev zlib1g-dev libncurses5-dev`
Source: https://github.com/sstephenson/ruby-build/wiki

Also updated `pkgs` to `names` to reflect the correct style using the latest stable salt release.

Thx to @simple10 for ruby troubleshooting assistance. "Oh yeah... rbenv install is provided by ruby-build plugin" :-)