Adapt imports to moved path in slapos.recipe.build

In slapos.recipe.build >= 0.57, code slapos/recipe/__init__.py
(which is a legacy namespace) was moved to slapos/recipe/utils.py

Also version up slapos.recipe.build requirement to >= 0.57

rubygems.py

@@ -12,7 +12,7 @@ import six.moves.urllib as urllib
 
 from zc.buildout import UserError
 from zc.buildout.easy_install import allow_picked_versions
-from slapos.recipe import rmtree
+from slapos.recipe.utils import rmtree # from slapos.recipe.build
 from slapos.recipe.downloadunpacked import Recipe as Download
 
 strip = lambda x:x.strip()  # noqa

setup.py

@@ -31,7 +31,7 @@ setup(name=name,
           'six',
           'zc.buildout',
           'setuptools',
-          'slapos.recipe.build',
+          'slapos.recipe.build>=0.57',
       ],
       extras_require={
           'test': test_requires