Require PHP 7.1

This will be the last PHP requirement upgrade for a while, at least
until stable (and therefore until the next major release).

We have decided to do this now, for the following reasons:
- We want to support MariaDB (and the compatible release of
  doctrine/dbal requires 7.1 as well).
- We prefer to upgrade to Laravel 5.6 sooner rather than later.
- Using the PSR-15 middleware standard is easier this way, as we do
  not have to switch from zend-stratigility to another PSR-15
  implementation. (Stratigility v3, which implements the final
  standard, requires 7.1.)

framework/core/composer.json

@@ -20,7 +20,7 @@
         "docs": "http://flarum.org/docs"
     },
     "require": {
-        "php": ">=7.0",
+        "php": ">=7.1",
         "dflydev/fig-cookies": "^1.0.2",
         "doctrine/dbal": "^2.5",
         "components/font-awesome": "^5.0.6",