Log Message:
Update lang/elixir to 1.6.1.

1. Enhancements

Elixir
- [DynamicSupervisor] Implement child_spec/1 for DynamicSupervisor
- [Kernel] Raise better error messages on invalid map syntax

2. Bug fixes

Elixir
- [Code.Formatter] Only rearrange not in operator if explicitly
  opted-in
- [Code.Formatter] Ensure do blocks do not exceed line length on calls
  with a single argument
- [Collectable] Support bitstrings in Collectable and
  for-comprehensions (regression in v1.6.0)
- [GenServer] Do not override user own @opts attribute
- [Enum] Reintroduce zipping of any enumerable of enumerables in
  Enum.zip/1 (regression in v1.6.0)
- [Macro] Reorder kw blocks in Macro.to_string/1 to avoid warnings
- [Protocol] Fix protocol consolidation when some chunks may be
  missing
- [Stream] Reintroduce zipping of any enumerable of enumerables in
  Stream.zip/1 (regression in v1.6.0)
- [Supervisor] Do not override user own @opts attribute
- [Supervisor] Add @spec to second clause of start_link/2

ExUnit
- [ExUnit.Case] Reintroduce :case in ExUnit setup/setup_all/test
  context