Update PHP requirement

There are some very clear PHP 7 dependencies now; just bumping the documentation to reflect this.

Author: bradjones1

doc/index.rst

@@ -4,7 +4,7 @@ Welcome to the OpenStack SDK for PHP!
 Requirements
 ------------
 
-* PHP 5.6
+* PHP 7
 * cURL extension
 
 Installation
@@ -58,4 +58,4 @@ If you'd like to contribute to the project, or require help running the unit/int
 .. _Composer installation instructions: `https://getcomposer.org/doc/00-intro.md`
 .. _Github repo: `https://github.com/php-opencloud/openstack`
 .. _mailing list: `https://groups.google.com/forum/#!forum/php-opencloud`
-.. _contributing guidelines: `https://github.com/php-opencloud/openstack/blob/master/CONTRIBUTING.md`
+.. _contributing guidelines: `https://github.com/php-opencloud/openstack/blob/master/CONTRIBUTING.md`