Provide update only for next major version, don't skip versions

[PVince81]

The updater app should not skip versions.
If you're running on OC 5 the updater should propose updating to OC 6, not OC 7.

Currently on OC 5 it will update to OC 7 which might break because it's unsupported.

One problem now is that we probably don't want to fix/touch the updater shipped with OC 5.
But this issue still needs to be fixed for future OC versions.

@VicDeo

[VicDeo]

@PVince81 #17 (comment)

[VicDeo]

and owncloud/appstore-issues#4

[VicDeo]

If you're running on OC 5 the updater should propose updating to OC 6, not OC 7.
in short: fix the link that is sent by appstore as described in owncloud/appstore-issues#4 and you even be capable to install wordpress while upgrading OC. For any version. Even for 4.x

[VicDeo]

related to core https://github.com/owncloud/core/blob/master/lib/private/updater.php#L69 or appstore owncloud/appstore-issues#4
Does not related to updater.