Changes for desktop client release version 1.6.1

hey, this contains the changes that should go live ASAP regarding the desktop client 1.6.1 release.

Please check how we can disable the display of the test version. For the next time, there is no test version that should show up on the install page, until we release 1.7 beta or so. Thanks!
@dragotin This is giving me a 404. Is this the correct construction? Output is this atm: https://download.owncloud.com/desktop/stable/mirall-1.6.1.tar.bz2.asc