Fixed casting bug

When executing `watson -c 2`, error occurred as follows.

```sh
/Users/alpaca-tc/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/watson-ruby-1.5.0/lib/watson/parser.rb:284:in `+': String can't be coerced into Fixnum (TypeError)
        from /Users/alpaca-tc/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/watson-ruby-1.5.0/lib/watson/parser.rb:284:in `block in parse_file'
...
```