Pin shellcheck version to v0.6.0

Author: practicalswift

.travis/lint_04_install.sh

@@ -9,3 +9,7 @@ export LC_ALL=C
 travis_retry pip install codespell==1.13.0
 travis_retry pip install flake8==3.5.0
 travis_retry pip install vulture==0.29
+
+SHELLCHECK_VERSION=v0.6.0
+curl -s "https://storage.googleapis.com/shellcheck/shellcheck-${SHELLCHECK_VERSION}.linux.x86_64.tar.xz" | tar --xz -xf - --directory /tmp/
+export PATH="/tmp/shellcheck-${SHELLCHECK_VERSION}:${PATH}"