Drop support for CentOS 6 and Ubuntu 12.04 LTS

[geerlingguy]

Both of these older distros are a huge burden, and cause quite a bit of maintenance and extra testing (and mental drain) to support. I've now been supporting a matrix of 4 OSes, and especially because of Debian's batty decisions to change entire directory structures in 10 different ways, I've always wanted to cut off the older versions of both OSes since the release of 14.04 and 7.

Additionally, Ubuntu 16.04 is soon to be released, and when that happens, about 99% of the tooling I rely on to not have to maintain my own Ubuntu ecosystem of repos will be dropping support for Precise entirely. Some of ondrej's PPAs have already dropped support, and others will follow.

Timeline:

• Ubuntu 12.04 LTS: Unsupported in April 2017... I'd like to drop it maybe by end of 2016?
• CentOS 6: Unsupported in Q2 2017... I'd like to drop it ASAP. Switch Apache to use FPM instead of mod_php #346 makes CentOS 6 support even more marginal than it already was.

[geerlingguy]

Meh, I wrote this more out of momentary frustration. I will be dropping support for LTS OS releases when they hit end-of-life, but no need to keep an issue festering to remind me of it :)