Update deploy-ubuntu.md

Added clarification for benefit of those who are following this guide using Ubuntu 12.04 or Ubuntu 10.04. When installing nginx-extras, the command sudo add-apt-repository 'deb https://oss-binaries.phusionpassenger.com/apt/passenger trusty main' is specific to the version of Ubuntu. For instance, under Ubuntu 12.04, the command should be: sudo add-apt-repository 'deb https://oss-binaries.phusionpassenger.com/apt/passenger precise main'
Merge pull request #141 from jyek/ubuntu-install-clarification

Update deploy-ubuntu.md