Compatibility with Ecto ~> 3.2, ULID.t, formatter

[woylie]

I needed this to work, so I created my own fork and cherry-picked what I needed and made some changes to make it work on the CI.

• fixes compatibility issue with Ecto ~> 3.2 (cherry-picked from Fix warning when using ecto 3.5. #7)
• adds ULID type (from Adding type t #6)
• bumps Ecto requirement to ~> 3.2
• requires Elixir 1.7 (because of newer Ecto versions)
• updates CI configuration
• formats code with mix format

[woylie]

I don't want to rush anybody, but do you intent to merge any of the pull requests addressing this issue? The first PR addressing it is more than a year old.

[woylie]

See #10.