allow aliases in config files

[dalyons]

we'd like to be able to use aliases in the slashdeploy.yml files, like:

environments:
  production: 
    aliases:
      - prod
    continuous_delivery: &cd
      ref: refs/heads/master
      required_contexts:
        - "ci/circleci: docker_image"
        - "ci/circleci: bundle_audit_check"
        - "ci/circleci: cucumber"
        - "ci/circleci: prevent_unsafe_migrations"
        - "ci/circleci: rspec"
  staging:
    continuous_delivery:
      <<: *cd
    aliases:
      - stage

However, safe_load doesnt allow aliases by default:

Psych::BadAlias: Unknown alias: cd

Change it to let us use them