pulp: use new environ feature of zc.recipe.egg:develop

This works around https://github.com/buildout/buildout/issues/159

component/pulp/buildout.cfg

@@ -13,19 +13,14 @@ recipe = slapos.recipe.build:gitclone
 repository = https://gitlab.com/jerome-nexedi/pulp-or.git
 branch = master
 git-executable = ${git:location}/bin/git
-develop = true
-
-[pulp-build]
-recipe = zc.recipe.egg:develop
-setup = ${pulp-repository.git:location}
 
 [pulp-env]
 <= numpy-env
 PATH=${glpk:location}/bin
 
 [pulp]
-dependencies = ${pulp-build:setup}
-recipe = zc.recipe.egg:custom
+recipe = zc.recipe.egg:develop
+setup = ${pulp-repository.git:location}
 egg = pulp
 environment = pulp-env
 setup-eggs = ${numpy:egg}