Add environment check for versions not supporting PHP 7.3

Note: Testing this issue requires, for some points, a running version of PHP 7.3 (at the time of writing, current version is PHP 7.3RC2). Note2: Alternatively, if finding a working version is hard... the lib/environmentlib.php file can be edited replacing the two existing calls to phpversion() by, simply: '7.3.0' (quotes included). Using PHP 7.0.x - All upgrade work Install a Moodle 3.1.x site (MOODLE_31_STABLE) Upgrade to 3.3.x (MOODLE_33_STABLE) Upgrade to 3.4.x (MOODLE_34_STABLE) Upgrade to 3.5.x (MOODLE_35_STABLE) Upgrade to 3.6dev (master). Verify that all the upgrades above work without problem. Using PHP 7.0.x - Upgrade to 34 & 35 with PHP 7.3 don't work Install a Moodle 3.1.x site (MOODLE_31_STABLE) Switch to PHP 7.3 Try to upgrade to 3.3.x (MOODLE_33_STABLE) Verify that upgrade doesn't work and you get the "PHP version 7.3 is not supported" message. Try to upgrade to 3.4.x (MOODLE_34_STABLE) Verify that upgrade doesn't work and you get the "PHP version 7.3 is not supported" message. Try to upgrade to 3.5.x (MOODLE_35_STABLE) Verify that upgrade doesn't work and you get the "PHP version 7.3 is not supported" message. Try to upgrade to 3.6dev (master). Verify that the upgrade works without problem.