/usr/bin integration

Rubies are now installed into their own directories that are not in PATH. This safe, but it's not very user-friendly, especially not for users who only intend to install 1 Ruby version on their system.

We should have some kind of way to provide /usr/bin/{ruby,gem,bundle,irb}. This will conflict with the system Ruby package, but _c'est la vie_.

On Debian/Ubuntu, we should integrate with the alternatives system.

## How would this work? A proposal

We provide an optional `fullstaq-ruby-system-integration` package. This package contains the scripts /usr/lib/fullstaq-ruby/system-integration/bin/{ruby,gem,bundle,irb}.

These scripts simply call `/usr/bin/rbenv exec ruby|gem|bundle|irb`.

This package integrates with the alternatives system to ensure that /usr/bin/{ruby,gem,bundle,irb} are symlinked to the corresponding script in /usr/lib/fullstaq-ruby/system-integration/bin.