use sql to better query for billable students

Before we were using the ruby select method to filter through all
the students who's latest payment was within a certain amount of
time. This created an n + 1 situation where we were querying for
students, then looping through and querying for each student's
payments. Not ideal. Now it's all done in one fairly complex SQL
statement.
Merge pull request #78 from epicodus/billable-sql

use sql to better query for billable students