Update .aggregate() statements

[justincy]

This is due to the Mongoid 5 upgrade. According to another migration doc, aggregate only accepts an array.

Ruby docs for .aggregate().

Instances of aggregate:

$ grep -rn aggregate app
app/models/subject.rb:188:    self.collection.aggregate(agg).inject({}) do |h, p|
app/models/subject.rb:196:    self.collection.aggregate([
app/models/classification.rb:126:    self.collection.aggregate(agg).inject({}) do |h, p|
app/models/user.rb:219:    self.collection.aggregate(agg).inject({}) do |h, p|

[justincy]

I don't actually know if that code needs to be changed or not. Just leaving it here as a task that needs to be reviewed.

[tfmorris]

They all look like arrays to me, but I'll leave it open for @benwbrum