Exclude pre-release versions as per SemVer

As SemVer can be used apps could define a release like "10.0.0-alpha". This is something that we don't support at the moment in the server and we should filter all prereleases.

Ref https://github.com/nextcloud/server/pull/2307#issuecomment-262911588



Signed-off-by: Lukas Reschke <lukas@statuscode.ch>