handle buildout `allow-picked-version` value

If deployment mode option is not provided, default
value is `not allow-picked-version`.

README.rst

@@ -46,7 +46,7 @@ ruby-executable
 
 deployment
     If set to ``true``, the version of each gem dependency must be provided in
-    ``gems`` option. Default value is ``false``. 
+    ``gems`` option. Default value is ``not allow-picked-versions``. 
 
 gem-options
     Extra options, that will be passed to gem executable. Example::

rubygems.py

@@ -13,11 +13,13 @@ import zc.buildout
 
 import six.moves.urllib as urllib
 
-from distutils.util import strtobool
 from slapos.recipe.downloadunpacked import Recipe as Download
+from zc.buildout.easy_install import allow_picked_versions
 
 strip = lambda x:x.strip()  # noqa
 
+is_true = ('false', 'true').index
+
 
 class Recipe(object):
     """zc.buildout recipe for compiling and installing software"""
@@ -45,8 +47,8 @@ class Recipe(object):
         # Allow to define specific ruby executable. If not, take just 'ruby'
         self.ruby_executable = options.get('ruby-executable', 'ruby')
 
-        deployment = options.get('deployment', 'false')
-        self.deployment = bool(strtobool(deployment))
+        deployment = options.get('deployment')
+        self.deployment = is_true(deployment) if deployment else not allow_picked_versions()
         self.gem_regex = re.compile(r'\s+([\w\-_.]+) \((<|~>|>=) '
                                     r'((\d+\.)*\d+)(, (>=) ((\d+\.)*\d+))?\)')