Modify event count

This is related to https://github.com/mephux/ruby-nessus/issues/5
```
Enumerable.count returns size if it exists otherwise
it counts the items through enumeration. This was
causing errors under 1.9.3.
http://ruby-doc.org/core-1.9.3/Enumerable.html
```
I modified event_count to use .count instead of .to_i.