Update ubuntulinux.md

Hi - Installing Docker on my Ubuntu 14.04 LTS failed on the line 

   source /etc/bash_completion.d/docker.io

since this implicitely assumes one is running the BASH shell.  Since I was running ZSH I got the error 

   ~ > source /etc/bash_completion.d/docker.io
   /etc/bash_completion.d/docker.io:679: command not found: complete
   /etc/bash_completion.d/docker.io:680: command not found: complete

I thought we should either update the instructions or modify them to avoid BASH assumptions.  :)

Alan Thompson