Add apt-get update, hope we can install unzip now

.gitlab-ci.yml

 before_script:
   - rm -rf /usr/local/go
+  - apt-get update -qq
   - apt-get install -y curl unzip bzip2
   - curl -O https://storage.googleapis.com/golang/go1.5.1.linux-amd64.tar.gz
   - echo '46eecd290d8803887dec718c691cc243f2175fe0  go1.5.1.linux-amd64.tar.gz' | shasum -c -