Update to use elixir 1.5 official image

[jeammyb]

Fixes #1

[dantswain]

@jeammyb Thanks for the contribution. This requires a couple changes to work.

1. Update the postgrex dependency:

# in mix.exs
{:postgrex, "~> 0.12.2"}

Then mix deps.get so that it updates mix.lock.

2. We need to set MIX_HOME and HEX_HOME in the compose file so that they are persistent:

    # in the environment section of the web service
    HEX_HOME: /beanie/.hex/docker
    MIX_HOME: /beanie/.mix/docker

[jeammyb]

ok, I will work on that.

[jeammyb]

Hi @dantswain, I Updated the postgrex version, and the environment of web in the compose file.
To have the database working I had to execute these commands in the Web container: mix ecto.create, mix ecto.migrate, is that normal?