Fix to Issue #2 Top Month Always Empty

[im-mattbrown]

This change to /lib/archives_sidebar/lib/archives_sidebar.rb fixes issue #2. By adding -1 here:
month = (entry.month.to_i**-1**%12)+1
this sets the correct month from an array as January would be at index zero and previously it was selecting from index 1 as 0%12= 0(january) but 1%12=1(february). In this particular case the most recent month should be September[index 8] but the logic made it select October[index 9]

[nathanallen]

You are reasoning about this correctly, and your description is sound. Please just add +1-1+2-2+0000 to your solution and we will merge it in.