Support for Ecto 3

[anuragjain67]

To upgrade elixir version we need to upgrade ecto as well and for this ecto_mnesia should support ecto3 version.

[anuragjain67]

I have just looked at these tickets. (Sorry for recreating another ticket)
#76
#72

@AndrewDryga Still looking for your inputs on this.

[KushanChamindu]

I am also interested in this issue because when my project compiling it shows a lot of warning.
ex:

warning: "not expr1 in expr2" is deprecated. Instead use "expr1 not in expr2" if you require Elixir v1.5+, or "not(expr1 in expr2)" if you have to support earlier Elixir versions lib/ecto/changeset.ex:1409