OP-5586 - Create a Ruby (rbenv) oriented 'env' script to use as a shebang 

When defining a Ruby script to execute, you specify the following as the shebang:

`#!/usr/bin/env ruby`

But you have no say as to which version of Ruby is executed here.  Unfortunately the super neat trick proposed by @mfischer-zd does not work on Linux:

`#!/usr/bin/env RBENV_VERSION=1.9.3-p484 ruby`

This PR introduces a `/usr/local/bin/env_ruby` env wrapper script which allows a specific Ruby version to be defined in the shebang, like so:

`#!/usr/local/bin/env_ruby 1.9.3-p484`

The PR simply drops the `/usr/local/bin/env_ruby` wrapper on disk which means existing Ruby scripts need to be updated to use the new shebang.

/cc @zendesk/chef @mfischer-zd 

### References
 - Jira link: https://zendesk.atlassian.net/browse/OP-5586