Use rbenv

[lcreid]

For people who are deploying to shared servers, they still need to version their Ruby. So this box should come with rbenv and the latest Ruby, but allow users to install previous Rubies.

This brings up a few questions:

• Should we pre-install a few of the recent, supported versions of Ruby?
• Some versions of gems are incompatible with some versions of Linux, e.g. json (and the net says libxml is often a culprit). So complete portability of apps may be a challenge. That probably means don't knock yourself out to try to solve the whole problem
• Do we need to worry about installing gems locally? rbenv probably does that
• How much does this change how we issue commands? Do we have to bundle exec in front of everything?
• What implications does this have for our deploy scripts?

We're going with rbenv rather than rvm because rbenv is less intrusive, and Bundler provides what rvm provides that rbenv doesn't.

[lcreid]

rbenv is now included. I really should test to see what it does to using the box as before, i.e. without rbenv.

[lcreid]

I should note that this is also needed for supporting legacy apps: they may need older versions of Ruby, and using either rbenv or rvm is the best way to deal with different Ruby versions.

[lcreid]

It also seems okay to use it the "old" way, without .rbenv.