End support for PHP 5.6

[dnl-blkv]

How to Reproduce

1. Try composer install with PHP 5.6 for the SDK

What Should Happen

1. composer install fails, because PHP 5.6 is not supported

What Happens

1. composer install succeeds
2. Examples/tests fail due to a PHP 5.6-specific bug

Notes

1. The SDK was not working with PHP 5.6 for some time, but the question was only raised once, and the questioning person was OK with switching to PHP 7.0
2. Therefore, in order to be sure we provide best possible SDK for PHP 7.0+, and to contribute to the migration of developers to PHP 7.0 we would like to end support of PHP 5.6
3. This would mean changes to the code style, such as adding scalar parameter and return type-hints or using other language features introduced in PHP 7.0

[OGKevin]

Closing in favor of #15