travis: update shellcheck URL

The latest build failed with this error:

```
You are downloading ShellCheck from an outdated URL!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 :(
```

.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"