Elixir recipe: Fix path problems when calling "erl" in requirement.

Since we use "which 'erl'" anyway, let's use the full path to the binary.

This solves a problem I have been having when trying to install elixir 0.10.1 with the latest version of erlang (R16B01 (erts-5.10.2)): I was getting "sh: erl: command not found" otherwise, even though "which 'erl'" returned the proper path.