Put in a temporary directory the .beam files generated by Elixir compiler

Hi Sebastian,

This PR put the generated .beam files in a temporary directory. On Linux, this directory is `/tmp/flycheck-elixir`

For now it may be ok to just put those .beam files in a temporary directory.
It is not trivial to accurately clean the .beam files because `elixirc` creates one .beam per module declared in a file. It is possible to declare several modules in the same file.

For example:

    defmodule Hello do
    end

produces `Elixir-Hello.beam`

Cheers,
syl20bnr