[gnome-calculator] Use the core18 based snapcraft container for the snap CI build

[Ken VanDine <kvandine src gnome org>]

commit 8f58d103d5929090dcbc3a5a5f7dc79cef6a0629
Author: Ken VanDine <ken vandine org>
Date:   Tue Jun 11 11:31:04 2019 -0400

    Use the core18 based snapcraft container for the snap CI build

 .gitlab-ci.yml | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 86144a09..36bc70c6 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -34,11 +34,7 @@ build:ubuntu:
 
 build:snap:
   stage: build
-  image: snapcore/snapcraft
-  before_script:
-    - apt-get update
-    - apt-get install -q -y --no-install-recommends software-properties-common
-    - add-apt-repository -yu ppa:ubuntu-desktop/gnome-3-26
+  image: ubuntudesktop/gnome-3-28-1804
   script: snapcraft
   allow_failure: true
   artifacts: