Provide clarity that yaml is parsed by Ruby/Psych

It is not obvious from our documentation that .gitlab-ci.yml is parsed by Ruby/Psych, in particular that values provided such as:

variables:
  var_a: 03334444456

Would result in var_a = 460474670 (the octal value, as the value begins '0'). Proper usage would be:

variables:
  var_a: '03334444456'

Ticket: https://gitlab.zendesk.com/agent/tickets/373542 Doc link: https://docs.gitlab.com/ee/ci/variables/