Added CI verification.

.gitlab-ci.yml

+image: privacybydesign/irma:latest
+
+stages:
+  - verify
+
+before_script:
+  - set -euxo pipefail
+  - ln -s "$CI_PROJECT_DIR" "/tmp/pbdf"
+  - cd /tmp/pbdf
+
+verify:
+  stage: verify
+  artifacts:
+    paths:
+      - ./*
+  script:
+    - irma scheme verify