Run elixir tests properly on travis

[gomoripeti]

We would like to test xprof with and without elixir.
Because we also would like to test with old Erlang versions (like OTP 16) that is not supported by any elixir version, we cannot use language: elixir.

A workaround seems to be to keep using language: erlang and manually download specific elixir version (try with kiex)