Fix dialyzer warnings

[rupurt]

$ mix dialyzer
==> erlex
Compiling 1 file (.yrl)
src/parser.yrl: Warning: conflicts: 25 shift/reduce, 0 reduce/reduce
Compiling 1 file (.xrl)
Compiling 2 files (.erl)
Compiling 1 file (.ex)
Generated erlex app
==> dialyxir
Compiling 56 files (.ex)
Generated dialyxir app
==> logger_file_backend
Compiling 1 file (.ex)
Generated logger_file_backend app
Finding suitable PLTs
Checking PLT...
[:compiler, :elixir, :kernel, :stdlib]
Looking up modules in dialyxir_erlang-21.2.5_elixir-1.8.1_deps-dev.plt
Looking up modules in dialyxir_erlang-21.2.5_elixir-1.8.1.plt
Finding applications for dialyxir_erlang-21.2.5_elixir-1.8.1.plt
Finding modules for dialyxir_erlang-21.2.5_elixir-1.8.1.plt
Checking 418 modules in dialyxir_erlang-21.2.5_elixir-1.8.1.plt
Finding applications for dialyxir_erlang-21.2.5_elixir-1.8.1_deps-dev.plt
Finding modules for dialyxir_erlang-21.2.5_elixir-1.8.1_deps-dev.plt
Copying dialyxir_erlang-21.2.5_elixir-1.8.1.plt to dialyxir_erlang-21.2.5_elixir-1.8.1_deps-dev.plt
Looking up modules in dialyxir_erlang-21.2.5_elixir-1.8.1_deps-dev.plt
Checking 418 modules in dialyxir_erlang-21.2.5_elixir-1.8.1_deps-dev.plt
Adding 48 modules to dialyxir_erlang-21.2.5_elixir-1.8.1_deps-dev.plt
Starting Dialyzer
[
  check_plt: false,
  init_plt: '/Users/me/workspace/logger_file_backend/_build/dev/dialyxir_erlang-21.2.5_elixir-1.8.1_deps-dev.plt',
  files_rec: ['/Users/me/workspace/logger_file_backend/_build/dev/lib/logger_file_backend/ebin'],
  warnings: [:unknown]
]
Total errors: 4, Skipped: 0
done in 0m1.08s
:0:unknown_function
Function Logger.compare_levels/2 does not exist.
________________________________________________________________________________
:0:unknown_function
Function Logger.Formatter.compile/1 does not exist.
________________________________________________________________________________
:0:unknown_function
Function Logger.Formatter.format/5 does not exist.
________________________________________________________________________________
:0:unknown_type
Unknown type: Logger.level/0.
________________________________________________________________________________
done (warnings were emitted)