PHP proxy should require at least PHP 5.6

[bsvensson]

Curently https://github.com/Esri/resource-proxy/blob/master/PHP/README.md states support down to PHP 5.4.2.
But versions prior to 5.6 are not supported from the PHP side - https://secure.php.net/supported-versions.php.

Also note that 5.6.30 was released yesterday 2017-01-19 and "this PHP 5.6 version is the last planned release that contains regular bugfixes. All the consequent releases will contain only security-relevant fixes, for the term of two years. PHP 5.6 users that need further bugfixes are encouraged to upgrade to PHP 7."

Not having to test old php versions should allow us to spend more time with php 7...

[bsvensson]

The readme has been updated. Note however, that this will soon change to PHP 7.1 as a minimum requirement - see #449.