Remove out-of-date "Elixir" package, add new one

Package for the Elixir programming language.

There was already a package with this name, but it is now out of date; there is a newer repo at a different URL which is a superset of the original repo.

The GitHub author of the original repo has given his approval to have his removed from Sublime Package Control.  See short conversation here: elixir-lang/elixir-tmbundle/issues/8