Fix the erlang package to use sh from the store, and not hardcode
/bin/sh.

This means that the elixir package no longer nondeterministically fails
to build, and allows for getting all the tests to pass as well.

These are the related bug reports:
 - https://debbugs.gnu.org/cgi/bugreport.cgi?bug=28034
 - https://debbugs.gnu.org/cgi/bugreport.cgi?bug=29655


Christopher Baines (2):
  gnu: erlang: Patch occurrences of /bin/sh in the source.
  gnu: elixir: Enable more tests and remove patch.

 gnu/local.mk                                  |   1 -
 gnu/packages/elixir.scm                       |  36 +--
 gnu/packages/erlang.scm                       |  21 ++
 .../elixir-disable-failing-tests.patch        | 284 ------------------
 4 files changed, 27 insertions(+), 315 deletions(-)
 delete mode 100644 gnu/packages/patches/elixir-disable-failing-tests.patch