[gnome-builder] build: add flatpak-beta channel

[Christian Hergert <chergert src gnome org>]

commit d25c40650d4f1876adbba69a976bd87b2498a59a
Author: Christian Hergert <chergert redhat com>
Date:   Tue Feb 19 15:37:05 2019 -0800

    build: add flatpak-beta channel

 meson_options.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/meson_options.txt b/meson_options.txt
index 6624227c4..0b5e487d3 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -7,7 +7,7 @@ option('with_safe_path', type: 'string', value: '', description: 'PATH variable
 
 option('with_channel',
           type: 'combo',
-       choices: [ 'other', 'flatpak-stable', 'flatpak-nightly' ],
+       choices: [ 'other', 'flatpak-stable', 'flatpak-beta', 'flatpak-nightly' ],
    description: 'The distribution channel for Builder',
 )