Replace deprecated term_to_binary with :erlang.term_to_binary

I tried weber on the latest elixir master and got an error that `term_to_binary` was not defined. I guess it has been deprecated and removed in master in favor of the version in `:erlang`

See https://github.com/elixir-lang/elixir/commit/ce61c60af56ff55e4e7ecf821b6b3d8a1dd8a492