mwan3: Fix json_load fails with some data

Sometimes the return value of `ubus -S call network.interface.wan status`
cause `json_load` to return `Failed to parse message data` error.

To avoid this, the JSON data always should be quoted with double quotes.

Signed-off-by: Evren Yurtesen <eyurtese@abo.fi>
Removed quoatation marks from commit heading
Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Update the version string from 2.8.1 (master) to 2.7.15 (openwrt-19.07)
(cherry picked from commit 94e0c788)

net/mwan3/Makefile

@@ -8,7 +8,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=mwan3
-PKG_VERSION:=2.7.14
+PKG_VERSION:=2.7.15
 PKG_RELEASE:=1
 PKG_MAINTAINER:=Florian Eckert <fe@dev.tdt.de>
 PKG_LICENSE:=GPLv2

net/mwan3/files/usr/sbin/mwan3

@@ -63,7 +63,7 @@ ifup()
 
 	status=$(ubus -S call network.interface.$1 status)
 	[ -n "$status" ] && {
-		json_load $status
+		json_load "$status"
 		json_get_vars up l3_device 
 	}