Ensure that '--version' flag works properly for root command (#595)

Make it so that, in the case that the root command is not runnable
but has subcommands, specifying a '--version' flag will still
run the "version" behavior.

command.go

@@ -685,7 +685,7 @@ func (c *Command) execute(a []string) (err error) {
 		return err
 	}
 
-	if helpVal || !c.Runnable() {
+	if helpVal {
 		return flag.ErrHelp
 	}
 
@@ -705,6 +705,10 @@ func (c *Command) execute(a []string) (err error) {
 		}
 	}
 
+	if !c.Runnable() {
+		return flag.ErrHelp
+	}
+
 	c.preRun()
 
 	argWoFlags := c.Flags().Args()