Elixir 1.10 support (#33)

* Adds Elixir 1.10.0 to the Travis CI matrix

* Ignores Erlang logger metadata keys that can't be encoded to JSON

Elixir 1.10 includes a better integration with the Erlang logger, this
means that the logger level, logger metadata, as well as all log
messages are now shared between Erlang and Elixir.

Some of the Erlang logger metadata doesn't have a valid representation
in JSON. This is a problem because when logger_json tries to encode them
it will fail

This commit updates logger_json to ignore the Erlang logger metadata keys
that can't be encoded to JSON.

Author: jbernardo95

.travis.yml

@@ -14,6 +14,8 @@ matrix:
       elixir: 1.7.3
     - otp_release: 22.1
       elixir: 1.9.2
+    - otp_release: 22.2
+      elixir: 1.10.1
 env:
   global:
     - MIX_ENV=test

lib/logger_json.ex

@@ -59,7 +59,7 @@ defmodule LoggerJSON do
   """
   @behaviour :gen_event
 
-  @ignored_metadata_keys ~w[ansi_color initial_call crash_reason]a
+  @ignored_metadata_keys ~w[ansi_color initial_call crash_reason pid gl mfa report_cb]a
 
   defstruct metadata: nil,
             level: nil,