[simple-scan] Update APT before installing deps

[Robert Ancell <rancell src gnome org>]

commit 4f1ac54fd854e6ec45d626f01a363c7d9af2e217
Author: Robert Ancell <robert ancell canonical com>
Date:   Thu Dec 7 10:04:33 2017 +1300

    Update APT before installing deps

 .gitlab-ci.yml | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 6563fb5..3830bf3 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -4,6 +4,7 @@ stages:
  - build
 
 before_script:
+  - apt update
   - apt install meson valac libgtk-3-dev libgusb-dev libcolord-dev libpackagekit-glib2-dev libwebp-dev 
libsane-dev gettext itstool
 
 build-simple-scan: