script: Lint Gitian descriptors with ShellCheck

ci/lint/04_install.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 #
-# Copyright (c) 2018 The Bitcoin Core developers
+# Copyright (c) 2018-2019 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -8,6 +8,7 @@ export LC_ALL=C
 
 travis_retry pip3 install codespell==1.15.0
 travis_retry pip3 install flake8==3.7.8
+travis_retry pip3 install yq
 
 SHELLCHECK_VERSION=v0.6.0
 curl -s "https://storage.googleapis.com/shellcheck/shellcheck-${SHELLCHECK_VERSION}.linux.x86_64.tar.xz" | tar --xz -xf - --directory /tmp/