update footer copyright to 2018

[sblaisot]

lp:1784789

[Pegasus-RPG]

It would probably be better to change the footer to a php file and have it do "Copyright 2001 - ".date('Y'); so we don't have to keep doing this. :)

[sblaisot]

All website is plain HTML so no PHP is possible here I think. Or that would make us redesign all website to use PHP which is another amount of work.
Either we use SSI (.shtml) if supported/configured on web server, but I'm not sure that's simple or use javascript which doen't seems to me like a good solution.

I prefer to stick on static date until someone else port the whole website to php if wanted.
I already update dates every year, so one more is not a problem:
mixxxdj/mixxx#838
mixxxdj/mixxx#1101
mixxxdj/mixxx#1452

[Pegasus-RPG]

Okay, good point. LGTM.