elixir/phoenix: change log level from :info to :error

Local testing shows this improves the performance of "plaintext"
benchmark from 11k to 28k responses per second by not printing log
entries like:

> 20:02:44.043 [info] GET /plaintext
> 20:02:44.043 [info] Sent 200 in 21µs
> 20:02:44.044 [info] GET /plaintext
> 20:02:44.044 [info] Sent 200 in 31µs
> 20:02:44.044 [info] GET /plaintext
> 20:02:44.044 [info] Sent 200 in 20µs

frameworks/Elixir/phoenix/config/prod.exs

@@ -28,7 +28,7 @@ config :hello, Hello.Repo,
 # Where those two env variables point to a file on
 # disk for the key and cert.
 
-config :logger, level: :info
+config :logger, level: :error
 
 # ## Using releases
 #