Upgrade to elixir-1.8.1

[bgracie]

now passing --enable-lenient to watchman ./configure as per
facebook/watchman#638 in order to solve (when building elixir-dev):

scm/Mercurial.cpp: In constructor 'watchman::Mercurial::infoCache::infoCache(std::__cxx11::string)':
scm/Mercurial.cpp:16:40: error: 'void* memset(void*, int, size_t)' clearing an object of non-trivial type 'struct watchman::FileInformation'; use assignment or value-initialization instead [-Werror=class-memaccess]
   memset(&dirstate, 0, sizeof(dirstate));
                                        ^
In file included from scm/Mercurial.h:10,
                 from scm/Mercurial.cpp:3:
./FileInformation.h:18:8: note: 'struct watchman::FileInformation' declared here
 struct FileInformation {
        ^~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make[1]: *** [Makefile:4446: scm/watchman-Mercurial.o] Error 1
make: *** [Makefile:1264: all] Error 2

error when building elixir-ci

      gcc -fno-strict-aliasing -Os -fomit-frame-pointer -g -DNDEBUG -Os -fomit-frame-pointer -g -DTHREAD_STACK_SIZE=0x100000 -fPIC -DUSE__THREAD -DHAVE_SYNC_SYNCHRONIZE -I/usr/include/ffi -I/usr/include/libffi -I/usr/include/python2.7 -c c/_cffi_backend.c -o build/temp.linux-x86_64-2.7/c/_cffi_backend.o
      c/_cffi_backend.c:2:10: fatal error: Python.h: No such file or directory
       #include <Python.h>
                ^~~~~~~~~~
      compilation terminated.
      error: command 'gcc' failed with exit status 1
  
      ----------------------------------------
  Command "/usr/bin/python2 -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-VmhNAV/cffi/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-record-2MECNl/install-record.txt --single-version-externally-managed --prefix /tmp/pip-build-env-YiWoxP --compile" failed with error code 1 in /tmp/pip-install-VmhNAV/cffi/
  
  ----------------------------------------
Command "/usr/bin/python2 -m pip install --ignore-installed --no-user --prefix /tmp/pip-build-env-YiWoxP --no-warn-script-location --no-binary :none: --only-binary :none: -i https://pypi.org/simple -- setuptools wheel "cffi>=1.1; python_implementation != 'PyPy'"" failed with error code 1 in None
The command '/bin/sh -c apk add py-pip   && pip install docker-compose   && docker-compose version' returned a non-zero code: 1

also unable to build it on master:

  Could not find a version that satisfies the requirement cffi>=1.1 (from versions: )
No matching distribution found for cffi>=1.1
You are using pip version 10.0.1, however version 19.0.3 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
The command '/bin/sh -c apk add py-pip   && pip install docker-compose   && docker-compose version' returned a non-zero code: 1

fixed with pinning version @ 1.23.2 as per
docker/compose#6617

[nicholasjhenry]

Thank you, @bgracie 👍 Maybe amend the commit message to remove the WIP?