Add etc gem to CI

This seems to not be installed by default on alpine linux.

.gitlab-ci.yml

@@ -15,7 +15,7 @@ before_script:
     libxslt libxslt-dev zlib-dev zlib
     ruby ruby-dev yaml yaml-dev
     libffi-dev build-base git nodejs
-  - gem install bundler --no-document
+  - gem install etc bundler --no-document
   - pwd
   - ls -l
   - bundle install
@@ -45,4 +45,3 @@ pages:
   # the "pages" job is only to be run for the master branch
   only:
     - master
-