Feature: adds deployment mode

- adds deployment mode in recipe options
- checks errors message in tests

README.rst

@@ -44,6 +44,10 @@ version
 ruby-executable
     A path to a Ruby executable. Gems will be installed using this executable.
 
+deployment
+    If set to ``true``, the version of each gem dependency must be provided in
+    ``gems`` option. Default value is ``false``. 
+
 gem-options
     Extra options, that will be passed to gem executable. Example::