Update to require PHP 7.1+

[SimonFrings]

This changeset updates the project to require PHP 7.1+ and drop legacy PHP < 7.1 and HHVM as discussed in #219. I'm marking this as a BC break for anybody still stuck on very old PHP versions, but there's little chance this will affect any current projects otherwise.

This PR aims to contain the minimal changeset to update the PHP version requirement only. Follow-up PRs will update our APIs to leverage newer language features.

FYI: I used https://github.com/reactphp/cache/pull/58 as inspiration.

Builds on top of #170, #217, #220 and others