Include Calculations module instead of prepending

[ehutzelman]

Another gem (groupdate: https://github.com/ankane/groupdate by @ankane) also patches the ActiveRecord::Relation#calculate method and breaks when used with this library. Prepending this Calculations module prevents any other libraries from patching this method, and it looks like an include will have the same desired effect to patch existing out-of-the-box ActiveRecord code. I'm not sure if this is prepended to ensure nothing else is patching #calculate, and how important that may be.

I see the reasoning for patching #calculate by this library is helpfully defined in the patched method:
# Same as original except we don't perform PostgreSQL hack that removes ordering.

Comparing to the original activerecord #calculate implementation, the following lines are removed for count operations:

# PostgreSQL: ORDER BY expressions must appear in SELECT list when using DISTINCT
relation.order_values = [] if group_values.empty?

In my somewhat obscure edge case scenario where both of these gems are used (along with postgres and sqlserver), I need to choose which gem is going to win the monkey patching war for #calculate. Changing this from prepend to include allows me to make this choice by the sequence the gems are loaded in. The downside of this change is it could allow another gem to patch #calculate away from the intended version in this library (but hopefully that is not much of a threat, and maybe a feature, as it is in my case).

[wpolicarpo]

Can I ask you to rebase your branch so CI can run again?