Web installer: check for low PHP versions before executing

Web installer was not checking for unsupported PHP versions.
In those cases the script should terminate or the
installation will break (e.g. syntax errors for PHP4).

Installer::doEnvironmentChecks() no longer checks for PHP
version since it has already been done by the entry scripts
(both in cli and web modes)

Installer::MINIMUM_PHP_VERSION has been removed since it is
no longer used.

Message config-env-php-toolow is no longer used and it has
been removed

Change-Id: I9227868bd2bd4e13bad6b95ad581be3ada71c94e
Merge "Web installer: check for low PHP versions before executing"