Support ecto 3

[sashman]

Failed to use "ecto" (version 3.0.6) because
  bourne (version 1.0.0) requires ~> 2.0
  ecto_sql (version 3.0.4) requires ~> 3.0.6
  phoenix_ecto (version 4.0.0) requires ~> 3.0
  mix.lock specifies 3.0.6


Failed to use "ecto" (version 3.0.6) because
  bourne (versions 1.0.1 and 1.1.0) requires ~> 2.1
  ecto_sql (version 3.0.4) requires ~> 3.0.6
  phoenix_ecto (version 4.0.0) requires ~> 3.0
  mix.lock specifies 3.0.6

If you know what needs to change, I'm happy to help out with a PR