Updating Instructions for Ubuntu Build Dependencies

[nathanbrauer]

• Replaced "still ships" with "ships", as Ubuntu 14.04 LTS is EOL
• Added sudo to apt-get commands as required in most Ubuntu environments
• Replaced python-cairo with python3-cairo (python-cairo no longer exists resulting in E: Unable to locate package python-cairo error message).

[cebtenzzre]

I think you accidentally broke the "ships an official package" link. Also, could you rebase this on master? I'm working on a 2.10.2 release and this seems like it's worth including.

[nathanbrauer]

Fixed the auto-added back-ticks...

To be clear on the rebase, you want me to rebase master into this feature branch? (This feature branch was created from develop.)

Do you still want the pull request targeting develop? Or do you want a new pull request targeting master as well?

PS: A CONTRIBUTIONS file explaining pull request expectations would be helpful :)

[cebtenzzre]

I'd like you to edit the pull request to target master, squash your commits, and rebase the feature branch onto the tip of master. That way I don't need to cherry-pick it. I will merge master into develop eventually, so I don't need another PR.

[nathanbrauer]

There you go :)