Fix validate_cmd file resource parameter

The validate_cmd parameter is only valid on newer versions of puppet.
This module is supporting all 3.x versions of puppet. This commit only
applies the validate_cmd parameter to the file resource if it is
defined and adds documentation noting that it will not work on older
versons of puppet.

README.md

@@ -218,6 +218,7 @@ Ensure there's a newline at the end of the fragments.
 
 #####`validate_cmd`
 Ensure the destination file passes the following validation command.
+Only supported on Puppet >= 3.5.0.
 
 ######Example
 - validate_cmd => '/usr/sbin/apache2 -t -f %'

manifests/init.pp

@@ -185,10 +185,16 @@
       path         => $path,
       alias        => "concat_${name}",
       source       => "${fragdir}/${concat_name}",
-      validate_cmd => $validate_cmd,
       backup       => $backup,
     }
 
+    # Only newer versions of puppet 3.x support the validate_cmd parameter
+    if $validate_cmd {
+      File[$name] {
+        validate_cmd => $validate_cmd,
+      }
+    }
+
     # remove extra whitespace from string interpolation to make testing easier
     $command = strip(regsubst("${script_command} -o \"${fragdir}/${concat_name}\" -d \"${fragdir}\" ${warnflag} ${forceflag} ${orderflag} ${newlineflag}", '\s+', ' ', 'G'))