Fixed interval check

There is a race condition if server replies slower than the checkInterval time frame.

This example shows when it goes wrong and the pull request fixes it.

``` js
var check = require('httpcheck');
var assert = require('assert');
var http = require('http');

http.createServer(function(request, response) {
	setTimeout(function() {
		response.end();
	}, 1000);
}).listen(13532, function() {
	var checks = 0;
	check({
		url: 'http://localhost:13532',
		checkTries: 3,
		checkInterval: 500
	}, function() {
		checks++;
		assert(checks === 1);
	});
});
```