.travis.yml: Fix shellcheck URL

This patch fixes the following Travis build failure:

    You are downloading ShellCheck from an outdated URL!

    Please update to the new URL:
    https://github.com/koalaman/shellcheck/releases/download/stable/shellcheck-stable.linux.x86_64.tar.xz

    For more information, see:
    koalaman/shellcheck#1871

    PS: Sorry for breaking your build :(

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

.travis.yml

@@ -24,7 +24,7 @@ addons:
 before_install:
   - sudo apt update || true
   - sudo apt install -y gcc-multilib g++-multilib
-  - curl -L "https://storage.googleapis.com/shellcheck/shellcheck-stable.linux.x86_64.tar.xz" | tar -xJ
+  - curl -L "https://github.com/koalaman/shellcheck/releases/download/stable/shellcheck-stable.linux.x86_64.tar.xz" | tar -xJ
   - sudo cp shellcheck-stable/shellcheck /usr/local/bin/
   - CFLAGS="-Werror"
   - CXXFLAGS="-Werror"