Point to 7-0-stable in installation docs

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Bump shell version

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'update-guide-7-0' into 'master'

Installation guide for 7.0

* Changed branch to 7-0-stable
* Bump gitlab-shell version to 1.9.6