Use temporary SNAPSHOT URL for 18.06

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>

options.conf.local

 release=Neumann
 version=testing
-openwrt_release=18.06
-base_url=https://downloads.openwrt.org/releases/$lede_release/targets/
-base_url=https://downloads.openwrt.org/snapshots/targets/
+openwrt_release=18.06.0-rc1
+base_url=https://downloads.openwrt.org/releases/18.06-SNAPSHOT/targets/
 communities_git=https://github.com/libremesh/network-profiles.git
 communities_dir=communities
 tmp_dir=tmp