Removed deprecation warnings for new elixir versions

[kuffel]

Hello,

thanks for this nice library. My pull-request is about removing deprecated warnings, support for elixir 1.4 and updating travis.yml to compile with --warnings-as-errors flag.

Since Elixir 1.4. is not even getting anymore security patches i think it can be remove. Please give me feedback if Elixir 1.4. support is still desired.

[ntrepid8]

Thanks for the pull-request! As you say, it's probably time to move on from Elixir 1.4 and only support the active versions.

[kuffel]

Thanks for the feedback. Can this be merged or is there anything missing?