Do not offer an update if PHP version doesn't match

[PVince81]

The updater (or update server ?) should not propose an update if the PHP version of the new ownCloud version is not compatible with the current one.

The goal is to avoid being in a "stuck" situation where someone might not be able to update their PHP version (shared hoster?) but then the source code has already been replaced and they have to restore their backups.

@VicDeo

[VicDeo]

@PVince81 err. I'd say even more: check all requirements against update server.

[ghost]

This was implemented for quite some time for PHP 5.4 and Windows in #107

But such hardcoded stuff is quite inflexible so the check on update server side is probably the best approach.

[VicDeo]

@PVince81 what do you think about extending update server with appinfo.xml-alike <dependencies> tag?

[VicDeo]

something like that:

<owncloud>
     <version>9.0.4</version>
     <versionstring>ownCloud 9.0.4</versionstring>
     <url>https://download.owncloud.org/community/owncloud-9.0.4.zip</url>
    <web>https://doc.owncloud.org/server/9.0/admin_manual/maintenance/upgrade.html</web>
    <dependencies>
        <php min-version="5.4" max-version="7.0" />
    </dependencies>
</owncloud>

[PVince81]

@VicDeo more like adding a new field to the update server's PHP config