[gnome-sdk-images] Revert "Change the Freedesktop-sdk runtime version to 18.08"

[Jordan Petridis <jpetridis src gnome org>]

commit 669ece83a6a5a53d63325f004685cc95a5d89212
Author: Jordan Petridis <jordanpetridis protonmail com>
Date:   Wed Aug 8 18:25:31 2018 +0000

    Revert "Change the Freedesktop-sdk runtime version to 18.08"
    
    This reverts commit 6cddad9ad24924f2a2b88e4155fec57c22347f2c

 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/Makefile b/Makefile
index 1171870..039d271 100644
--- a/Makefile
+++ b/Makefile
@@ -8,7 +8,7 @@ REPO   ?= repo
 # SDK_RUNTIME_VERSION: The org.freedesktop.BaseSdk and platform version to build against
 #
 SDK_BRANCH=master
-SDK_RUNTIME_VERSION=18.08
+SDK_RUNTIME_VERSION=unstable
 
 ifeq ($(ARCH),arm)
 LIB=lib/arm-linux-gnueabihf