Update to allow scrivener_ecto 2.x.x and ecto 3.x.x

v2.0.0

• Relax the requirements for scrivener_ecto to allow 1.x and 2.x
  versions - thereby also allowing ecto 3.x,
  resolves issue 4

• README example without a Repo module no longer references
  a Repo module confusingly - PR

• Formatting introduced with mix format

• Linting the README.md file

• Add example in README.md of adding adapter: Ecto.Adapters.Postgres in the
  use statement when creating a Repo module as is required
  for Ecto 3+.

• Elixir 1.4+ is required for scrivener_ecto 2.x