Fix offset beat group beaming

Fixes a beaming case that got broken during the massive amounts of PR merging/rebasing :) At some point the following case got broken, and the last two 8th notes weren't being beamed. Now fixed.

![image](https://cloud.githubusercontent.com/assets/1631625/3892892/24ce1a0a-2236-11e4-9eab-a4d534d0a145.png)