[gtranslator] Use flatpak gnome sdk 3.28

[Daniel Garcia Moreno <danigm src gnome org>]

commit f6181ecce786f1dc6d2e72068ffb339de20c8718
Author: Daniel García Moreno <danigm wadobo com>
Date:   Tue Sep 18 11:08:08 2018 +0200

    Use flatpak gnome sdk 3.28
    
    The gnome master runtime is a bit unstable right now so to get a
    buildable image I'm moving back to 3.28

 build-aux/flatpak/org.gnome.Gtranslator.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/build-aux/flatpak/org.gnome.Gtranslator.json b/build-aux/flatpak/org.gnome.Gtranslator.json
index 64f1f0de..d73d51bb 100644
--- a/build-aux/flatpak/org.gnome.Gtranslator.json
+++ b/build-aux/flatpak/org.gnome.Gtranslator.json
@@ -1,7 +1,7 @@
 {
     "app-id" : "org.gnome.Gtranslator",
     "runtime" : "org.gnome.Platform",
-    "runtime-version" : "master",
+    "runtime-version" : "3.28",
     "sdk" : "org.gnome.Sdk",
     "command" : "gtranslator",
     "tags" : [