Fix php compatibility to php5.6 [WIP]

[EugeneBogun]

Issue

After run ./vendor/bin/phpcs -p src --standard=PHPCompatibility --runtime-set testVersion 5.6- --report=source I got next errors

PHP CODE SNIFFER VIOLATION SOURCE SUMMARY
----------------------------------------------------------------------
STANDARD  CATEGORY            SNIFF                              COUNT
----------------------------------------------------------------------
PHPCompa  Operators           New operators t_coalesce found     26
PHPCompa  Function declar  New param type declarations bool   2
PHPCompa  Keywords            Forbidden names and found          1
----------------------------------------------------------------------
A TOTAL OF 29 SNIFF VIOLATIONS WERE FOUND IN 3 SOURCES
----------------------------------------------------------------------

Time: 45.79 secs; Memory: 26Mb

Changelog

• Raised mandatory php version in composer.json to 5.6
• Added phpcompatibility/php-compatibility for check code compatibility
• Added PHPCompatibility rule to phpcs.xml
• Fixed all compatibility issues