Update SDK to PHP 8

Merge pull request #21 from sivaschenko/php8

PHP8 сompatibility, Stock ticket STK-68866

Adding support for PHP 7.4

This release allows installation with newer phpunit and adjusts the tests to be compatible, also enabled tests for all 4 versions (PHP 7.1, 7.2, 7.3, 7.4 on travis)

Files API support

Added support for Files API

Bug fixes

Merge pull request #11 from sivaschenko/patch-2

Travis configuration to run tests on pull requests

Version update to 1.1.1

Rollup of changes since last merge

2019 release

• Includes custom PHP_CodeSniffer module for linting
• Removed dependency on ImageMagick
• Added missing license and other files