Remove out-of-date "Elixir" package, add new one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There was already a package with this name, but it is now out of date;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:

https://github.com/elixir-lang/elixir-tmbundle/issues/8
Merge pull request #1203 from mmorearty/patch-elixir

Remove out-of-date "Elixir" package, add new one