Performance improvement in add_set

[scottley]

I noticed that CCMatrix#add_set was taking a very long time. The culprit was all the increment calls for each pair that were generating tons of activity on the redis connection. By encapsulating them in a pipeline, add_set executes 2-3 times faster.

[travisbot]

This pull request passes (merged ce8612a9 into 15ad976b).