Remove dependency on maybe package

[tap349]

I'm using towel package which has Maybe module - just like maybe package.
As a result generating release fails:

     cmd: MIX_ENV=prod mix release --quiet
  stdout: ==> Release failed, during .boot generation:
                  Duplicated modules:
              	'Elixir.Maybe' specified in towel and maybe

It looks like maybe is used in elasticsearch-elixir only once:

https://github.com/infinitered/elasticsearch-elixir/blob/master/lib/mix/elasticsearch.build.ex#L101

So it can be safely removed and replaced, say, with Kernel.get_in.

I understand that it's a rare case and it's unlikely that anyone else will run into this issue but still IMO it's always a good idea to remove dependency - especially if it can be easily replaced with functions from standard library.