Address deprecations of Kernel.<-/2 and Enum.first/1 in elixir v0.12.2
Merge pull request #159 from TanYewWei/elixir-0.12.2-deprecations

Address deprecations of Kernel.<-/2 and Enum.first/1 in elixir v0.12.2
Update postgrex
Ensure and in validations nest properly, closes #158
Distinct: add basic distinct to the query DSL (+ add to PG adapter)
Distinct: refactor group_by/distinct into BuilderUtil
Distinct: factor out common code for distinct/group_by in normalizer
Distinct: rework validator to ensure group_by leftmost express match the distinct expr
Distinct: validate distinct on order_by not on group_by
Distinct: rename escape_expr in escape_fields_and_var + Enum.first => List.first
Distinct: proposal for the documentation of