feeds.conf: update to new turris-os-packages workflow

Branch for-v4.0 was renamed to master and master to develop.

feeds.conf

@@ -2,7 +2,7 @@
 ## It is commented out because it is not feed but is used by compile_pkgs script.
 # openwrt https://git.openwrt.org/openwrt/openwrt.git;openwrt-18.06
 
-src-git turrispackages https://gitlab.labs.nic.cz/turris/turris-os-packages.git;master
+src-git turrispackages https://gitlab.labs.nic.cz/turris/turris-os-packages.git;develop
 src-git packages https://git.openwrt.org/feed/packages.git;openwrt-18.06
 src-git luci https://git.openwrt.org/project/luci.git;openwrt-18.06
 src-git routing https://git.openwrt.org/feed/routing.git;openwrt-18.06