chore(travis): update shellcheck-docker to v0.6.0 (spf13#839)

Author: umarcor

.travis.yml

@@ -20,7 +20,7 @@ matrix:
 
 before_install:
   - mkdir -p bin
-  - curl -Lso bin/shellcheck https://github.com/caarlos0/shellcheck-docker/releases/download/v0.4.6/shellcheck
+  - curl -Lso bin/shellcheck https://github.com/caarlos0/shellcheck-docker/releases/download/v0.6.0/shellcheck
   - chmod +x bin/shellcheck
 script:
   - PATH=$PATH:$PWD/bin go test -v ./...