Update system-tools-backend version to 2.6.1

[Luca Ferretti <elle uca libero it>]

The current 2.6.0 approved version has a build issue on some libc
version[1]. It seems that 2.6.1 fixes this issue.

Is it ok to update [2] and jhbuild (I'll do it)? The only addition in
.1 release is the i18n, no api/abi/other breaks or changes.

[1] https://bugs.freedesktop.org/show_bug.cgi?id=19604
[2] http://live.gnome.org/TwoPointTwentyseven/ExternalDependencies