Suggestion: remove unsupported PHP versions

[XWB]

PHP 5.5 and 7.0 are EOL.

[peter-gribanov]

I'm not sure that it is a good idea to discard PHP 5 in this project.
Of course, PHP 5 is outdated and must be abandoned. I would want to go to PHP 7 and use type hinting in the project. But i don't think it's a good idea to do it now. I would want that users of old PHP versions could use features of operands #169.
I plan to set the PHP 7.2 as the minimum requirement for the next major release 2.0. I think it will be in this year.

PS: PHP 5.6 is unsupported too.

[XWB]

I'm not sure that it is a good idea to discard PHP 5 in this project.

That's why I kept PHP 5.6 in the requirements.

I don't think open source projects should encourage the usage of old PHP versions, but I'd be happy to wait for the next major release though :)