Not compatible with Rails 4 count distinct

[godfat]

Without squeel:

User.select(:username).distinct.count

Generates:

SELECT DISTINCT COUNT(DISTINCT "users"."username") FROM "users"

With squeel:

SELECT DISTINCT COUNT(DISTINCT ) FROM "users"

Which would cause syntax error. Tested with Rails 4.1.8 and pg 0.17.1 (PostgreSQL 9.3.5)

[godfat]

This caused: https://github.com/charlotte-ruby/impressionist/issues/178