raise PHP minimum required version to 5.5

[stopfstedt]

currently, we test for PHP 5.4 - even tho we use 5.6 in production and 5.5 for development (vagrant).

besides, PHP 5.4 has reached end of life in September 2015.

let's drop the pretense and go at least up to PHP 5.5. this blocks the completion of #1300, see #1301.

[stopfstedt]

@jrjohnson @saschaben objections?

[saschaben]

No objections here.

[jrjohnson]

RHEL / CentOS 7 are still stuck on php 5.4:
http://mirror.centos.org/centos/7/os/x86_64/Packages/
My concern is that deploying Ilios on campus becomes much more difficult if you need to mess with the existing packages on these systems. I'm worried that campus IT would be very hesitant to deploy something which required working from a third part repo.

Some discussion on this here:
#1207

[jrjohnson]

Oh - scratch that entirely. Looks like something called SoftwareCollections is an officially supported and up to date method of getting this done.
https://access.redhat.com/products/Red_Hat_Enterprise_Linux/Developer/#rhscl=&dev-page=5

That is AWESOME! In that case my choice is to go straight to PHP 5.6 which is EOL at the end of 2016 and when PHP 7 is available from this supported source then we can move to that.

[jrjohnson]

Interesting. OSX 10.11 comes with php 5.5 and there is no official way to get to 5.6. For developer ease of use I think we should stick with PHP 5.5 in composer requirements, have travis test 5.5 and 5.6 and document 5.6 as our official version requirement (since 5.5 is EOL and only has security support until July 2016)