Fix mplex for backwards compatibility with older Ruby versions

Ruby 1.8.6 and earlier don't have the count function. Some OS don't have the latest ruby installation and the script bug:

```undefined method `count' for ["src/msgpack/rpc/caller.hmpl", nil]:Array```

Replaced line 144 : 
```ruby
(finputs + [fctx]).count("-")
```
 by 
```ruby
(finputs + [fctx]).select {|e| e == "-"}.size
```