remove -1 from SMS line item in invoice for enterprise plans

fix http://manage.dimagi.com/default.asp?145881#829111

Before:

![screen shot 2014-12-01 at 5 26 45 pm](https://cloud.githubusercontent.com/assets/1757035/5254541/5812ed1e-797f-11e4-9db4-d1b03f13f651.png)

After:

![screen shot 2014-12-01 at 5 26 33 pm](https://cloud.githubusercontent.com/assets/1757035/5254546/5cf788e4-797f-11e4-97b2-d7391ca6c886.png)