Support environment running older PHP releases.

[ItsAGeekThing]

From @ItsAGeekThing on September 3, 2017 1:43

Currently, openCad in its current state only supports PHP >= 5.5.
Unfortunately, a lot of hosting providers have not yet updated their offerings to include later versions of PHP. In order to put openCad in the hands of more people, it would be a good idea to provide support for environments running PHP < 5.5.

OpenCad requires the function password_hash which was introduced in PHP 5.5.
It shouldn't require too much effort to backport this individual function to expand compatibility with a wider range of environments.

Copied from original issue: phillf/openCad#24

[ItsAGeekThing]

#39