Bumping gaia.json for 2 gaia-central revision(s) a=gaia-bump

Truncated some number of revisions since the previous bump.

========

https://hg.mozilla.org/integration/gaia-central/rev/28db9b0d9d78
Author: Marcus Cavanaugh <m@mcav.com>
Desc: Merge pull request #14373 from mcav/land-945669

Bug 945669 - [email/POP3] Make FolderStorage.sliceOpenMostRecent not trigger connection establishment for POP3 folders other than the inbox; clean up tests, r=asuth

========

https://hg.mozilla.org/integration/gaia-central/rev/5ba059d3d99c
Author: Marcus Cavanaugh <m@mcav.com>
Desc: Bug 945669 - [email/POP3] Make FolderStorage.sliceOpenMostRecent not trigger connection establishment for POP3 folders other than the inbox; clean up tests, r=asuth

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "adb7896ababd1ccb828e11b05909c0e9ca22594e", 
+    "revision": "28db9b0d9d784dfc3dbb6fefe6c9d64703a1769b", 
     "repo_path": "/integration/gaia-central"
 }