varnish: Do not use erp5 branch to fetch cookbook.

Develop is bad anyway, but at least this fixes varnish software release in
master branch.

software/varnish/software.cfg

@@ -44,7 +44,7 @@ parts =
 [slapos.cookbook-repository]
 recipe = slapos.recipe.build:gitclone
 repository = http://git.erp5.org/repos/slapos.git
-branch = erp5
+branch = master
 git-executable = ${git:location}/bin/git
 
 [check-recipe]