Require PHP 5.5+ (#2495)

* Require PHP 5.5+

https://github.com/FreshRSS/FreshRSS/issues/2469#issuecomment-522255093
I think it would be reasonable to require PHP 5.5+ for the core of
FreshRSS after all.

As Frenzie said, WordPress currently requires PHP 5.6.20+, and it is the
most popular PHP application.

We would loose about 20% of the PHP servers according to
https://w3techs.com/technologies/details/pl-php/5/all but I expect this
number to drop fast after the release of CentOS 8 (CentOS accounts for
17% of Linux servers
https://w3techs.com/technologies/details/os-linux/all/all ).

Distributions:
* no impact on Ubuntu, Fedora, Alpine, OpenWRT, FreeBSD, OpenSuze,
Mageia, as all active versions have PHP > 7
* no impact on OpenSuze, Synology, as all active versions have PHP > 5.5
* we drop Debian 8 Jessie (-2020) - we keep supporting Debian 9 Stretch
(2017-06) - current is Debian 10 Buster
* we drop Red Hat 7 (-2024) - we keep supporting RHEL 8 (2019-05)
* we drop CentOS 7 (-2024) - we will support CentOS 8 (to be released
soonish)

When dropping older versions, I can better like when it is for a good
reason, and there is actually one with PHP 5.5, namely generators
(yield) https://php.net/language.generators.overview which I consider
using.

* Version note for JSON.php

* hex2bin

* Update .travis.yml

Co-Authored-By: Frans de Jonge <fransdejonge@gmail.com>

phpcs.xml

@@ -10,7 +10,6 @@
 	<exclude-pattern>./lib/http-conditional.php</exclude-pattern>
 	<exclude-pattern>./lib/JSON.php</exclude-pattern>
 	<exclude-pattern>./lib/lib_phpQuery.php</exclude-pattern>
-	<exclude-pattern>./lib/password_compat.php</exclude-pattern>
 	<!-- Duplicate class names are not allowed -->
 	<rule ref="Generic.Classes.DuplicateClassName"/>
 	<!-- Statements must not be empty -->