Revise GL CI parallelization and dependencies

[thisismydesign]

So if I understand it correctly we should have the following stages and jobs:

stages:
  - verification
  - deploy

lint:
  stage: verification

test:
  stage: verification

apiary-validate:
  stage: verification

code-quality:
  stage: verification

heroku-deploy:
  stage: deploy

gl-pages-deploy:
  stage: deploy

apiary-deploy:
  stage: deploy

This would result in maximum parallelization while making sure deploys only run if verifications are successful.

[gomorizsolt]

That's it, all the jobs under the verification stage run in parallel, whereas the deploy runs completely separately.

[gomorizsolt]

Resolved by #23.