.travis.yml: sync Erlang and Elixir versions, require Erlang/OTP 20.3
.travis.yml: sync Erlang and Elixir versions, require Erlang/OTP 20.3

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website