elixir: fix 1.7 build on linux

Changed files
+2
pkgs
development
interpreters
+2
pkgs/development/interpreters/elixir/generic-builder.nix
···
rm -vf rebar
ln -s ${rebar}/bin/rebar rebar
+
patchShebangs lib/elixir/generate_app.escript || true
+
substituteInPlace Makefile \
--replace "/usr/local" $out
'';