Logger crashes when backend comes from a dependency

[mobileoverlord]

Environment

• Elixir & Erlang/OTP versions (elixir --version): 1.10-dev (7f86ea3)
• Operating system: *

Current behavior

Configuring Logger backends to use a backend from a dependency causes Logger to crash when calling mix deps.get

Expected behavior

Logger backends should be able to be configured before code is loaded from the dependency.

Here is a project that reproduces this issue: https://github.com/mobileoverlord/test_logger