Upgrade to Elixir 1.6

[unnawut]

Important: Preferably this PR should be merged at the same time as ewallet/goban's #9 so everyone using Goban can enjoy eWallet in Elixir 1.6 with us.

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Issue/Task Number: T57

Overview

This PR upgrades eWallet to Elixir 1.6 along with necessary updates to the code and dependencies to support Elixir 1.6.

Changes

• Upgrade the dependency ex_machina in ewallet_db and local_ledger_db to 037d826 which fixes the issue with Elixir 1.6
• Upgrade rrene/credo to the latest master to support Elixir 1.6
• Fix some test assertions as warned by the new version of rrene/credo
• Upgrade Dockerfile to Elixir 1.6

Implementation Details

The upgrade replaces ex_machina's version with a reference commit because ex_machina hasn't released a new version yet despite requests #261 and #275.

Usage

Run mix deps.get then mix test. Tests should pass.

vagrant@ewallet:/vagrant$ elixir --version
Erlang/OTP 20 [erts-9.2] [source] [64-bit] [smp:1:1] [ds:1:1:10] [async-threads:10] [hipe] [kernel-poll:false]

Elixir 1.6.1 (compiled with OTP 19)

vagrant@ewallet:/vagrant$ mix test
==> local_ledger_db
.........................................................

Finished in 4.1 seconds
57 tests, 0 failures

Randomized with seed 716370
==> ewallet_db
..............................................................................................................................................................................................................................................................

Finished in 7.5 seconds
254 tests, 0 failures

Randomized with seed 716370
==> local_ledger
.........................

Finished in 2.2 seconds
25 tests, 0 failures

Randomized with seed 716370
==> ewallet
...............................................................................................................................................

Finished in 9.6 seconds
143 tests, 0 failures

Randomized with seed 716370
==> admin_api
........................................................................................................................................................................................

Finished in 12.0 seconds
184 tests, 0 failures

Randomized with seed 716370
==> ewallet_api
..........................................................................................................................................

Finished in 11.2 seconds
138 tests, 0 failures

Randomized with seed 716370
==> admin_panel
..

Finished in 0.07 seconds
2 tests, 0 failures

Randomized with seed 716370
==> url_dispatcher
......

Finished in 0.1 seconds
6 tests, 0 failures

Randomized with seed 716370

Impact

Usual deployment. No extra steps.

[unnawut]

Turns out I need to upgrade credo & fix some tests from the latest credo too

[unnawut]

Done. Ready for review.