Fix path of auto.sh in chruby.sh

I used your Dockerfile as a base for one of my Ruby projects when I noticed that chruby wasn't working. Shell session inside the container:

    $ chruby rbx
    /bin/bash: line 3: chruby: command not found
    $ source /etc/profile.d/chruby.sh
    /etc/profile.d/chruby.sh: line 3: /usr/local/chruby/share/chruby/auto.sh: No such file or directory
    $ cat /etc/profile.d/chruby.sh
    if [ -n "$BASH_VERSION" ] || [ -n "$ZSH_VERSION" ]; then
      source /usr/local/share/chruby/chruby.sh
      source /usr/local/chruby/share/chruby/auto.sh
    fi
    $ source /usr/local/share/chruby/chruby.sh
    $ source /usr/local/share/chruby/auto.sh
    $ chruby rbx
    $ which ruby
    /opt/rubies/rbx-2.2.9/bin/ruby

As shown a simple correction to the path in chruby.sh fixes it.