Unset RBENV stuff when starting mux sessions

Since tmuxinator is a Ruby application spawning tmux, starting it from an rbenv
Ruby shim will set RBENV_VERSION, RBENV_DIR, etc. This is an issue when projects
use .ruby-version files to specify the version of Ruby they're using, since
RBENV_VERSION will override the file contents.

This makes sure that RBENV_DIR and RBENV_VERSION gets unset before starting the
tmux session.