[csmith-project/creduce] c61c71: Do `apt-key add` before adding the `apt.llvm.org` ...

[Eric Eide <noreply@github.com>]

  Branch: refs/heads/master
  Home:   https://github.com/csmith-project/creduce
  Commit: c61c7122b5f85573eb95cb1850949d9c2a8420bb
      https://github.com/csmith-project/creduce/commit/c61c7122b5f85573eb95cb1850949d9c2a8420bb
  Author: Eric Eide <eeide@cs.utah.edu>
  Date:   2019-12-10 (Tue, 10 Dec 2019)

  Changed paths:
    M scripts/travis_deps.sh

  Log Message:
  -----------
  Do `apt-key add` before adding the `apt.llvm.org` repository.

As of LLVM 9, it seems that this order does not work:

  add-apt-repository -y \
      'deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-9 main'
  wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -

  # Yields:
  [...]
  Err:1 http://apt.llvm.org/bionic llvm-toolchain-bionic-9 InRelease
    The following signatures couldn't be verified because the public key
    is not available: NO_PUBKEY ...

But the opposite order does.  So do that.