feeds: fix target branch for turrispackages

Although on Gitlab primary branch is set to be master on build servers
it seems to be still test. This is probably because of our local git
cache. This forces build of master.

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-19.07
 
-src-git turrispackages https://gitlab.labs.nic.cz/turris/turris-os-packages.git
+src-git turrispackages https://gitlab.labs.nic.cz/turris/turris-os-packages.git;master
 src-git packages https://git.openwrt.org/feed/packages.git;openwrt-19.07
 src-git luci https://git.openwrt.org/project/luci.git;openwrt-19.07
 src-git routing https://git.openwrt.org/feed/routing.git;openwrt-19.07