fix log rotate

This patch fixes #9. 

However, I know that ruby's Logger would tell an error in multiprocess environments. See https://github.com/ruby/ruby/blob/trunk/lib/logger.rb#L630-L632

```ruby
        if FileTest.exist?("#{@filename}.#{i}")
          File.rename("#{@filename}.#{i}", "#{@filename}.#{i+1}")
        end
```

For example, an error occurs if a sequence is as followings:

```
Process A: FileTest.exist?("#{@filename}.#{i}") #=> true
Process B: FileTest.exist?("#{@filename}.#{i}") #=> true
Process A: File.rename("#{@filename}.#{i}", "#{@filename}.#{i+1}") #=> success
Process B: File.rename("#{@filename}.#{i}", "#{@filename}.#{i+1}") #=> error!
```

Moreover, if sequence is as below, it results in worse

```
Process A: FileTest.exist?("test.log.1") #=> true
Process A: File.rename("test.log.1", "test.log.2")
Process A: FileTest.exist?("test.log.0") #=> true
Process A: File.rename("test.log.0", "test.log.1")
Process B: FileTest.exist?("test.log.1") #=> true
Process B: File.rename("test.log.1", "test.log.2") #=> test.log.2 is overwriten by test.log.0 originally
Process B: FileTest.exist?("test.log.0") #=> false
```

Supporting log rotation in serverengine may not fit (that is, also in fluentd v11). What do you think?