Drop default "global" version in rbenv_script provider.

This will lead to errors if rbenv_script is invoked before any rbenv
Ruby versions are installed (and a global Ruby version is set).

References sous-chefs#33
References sous-chefs#43

README.md

@@ -487,7 +487,7 @@ notifies it.
       <td>
         A version of Ruby being managed by rbenv.
       </td>
-      <td><code>"global"</code></td>
+      <td><code>nil</code></td>
     </tr>
     <tr>
       <td>root_path</td>

providers/script.rb

@@ -45,15 +45,15 @@
 private
 
 def build_script_code
-  <<-SYSTEM_WIDE.gsub(/^ +/, '')
-    export RBENV_ROOT="#{rbenv_root}"
-    export PATH="${RBENV_ROOT}/bin:$PATH"
-    eval "$(rbenv init -)"
-
-    rbenv shell #{new_resource.rbenv_version}
-
-    #{new_resource.code}
-  SYSTEM_WIDE
+  script = []
+  script << %{export RBENV_ROOT="#{rbenv_root}"}
+  script << %{export PATH="${RBENV_ROOT}/bin:$PATH"}
+  script << %{eval "$(rbenv init -)"}
+  if new_resource.rbenv_version
+    script << %{export RBENV_VERSION="#{new_resource.rbenv_version}"}
+  end
+  script << new_resource.code
+  script.join("\n")
 end
 
 def build_script_environment

resources/script.rb

@@ -22,7 +22,7 @@
 actions :run
 
 attribute :name,          :kind_of => String, :name_attribute => true
-attribute :rbenv_version, :kind_of => String, :default => "global"
+attribute :rbenv_version, :kind_of => String
 attribute :root_path,     :kind_of => String
 attribute :code,          :kind_of => String
 attribute :creates,       :kind_of => String