Merge branch 'bump-gitlab-shell' into 'master'

Bump gitlab-shell to 2.6.8
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

For gitlab/gitlabhq#2635

See merge request !1939

GITLAB_SHELL_VERSION

-2.6.7
+2.6.8