Updates the URI to the Centos 7 PHP repository

The existing link to the ius-release-1.0-14.ius.centos7.noarch.rpm is now 404'ing. There is an update ius-release-1.0-15.ius.centos7.noarch.rpm to which I have updated the link in the docs.

Author: Jack Hughes

guides/v2.3/install-gde/prereq/php-centos-ubuntu.md

@@ -82,7 +82,7 @@ To install PHP 7.1 on CentOS 6 or 7:
 
 *CentOS 7*. Enter the following commands:
 
-  yum install -y http://dl.iuscommunity.org/pub/ius/stable/CentOS/7/x86_64/ius-release-1.0-14.ius.centos7.noarch.rpm
+  yum install -y http://dl.iuscommunity.org/pub/ius/stable/CentOS/7/x86_64/ius-release-1.0-15.ius.centos7.noarch.rpm
   yum -y update
 
 1. Install all [required PHP extensions]({{page.baseurl}}/install-gde/system-requirements-tech.html#required-php-extensions):