Fix OpenAPI v3.0.3 validation issues

[apd-bbrewington]

Fixes #67
(errors only, note the --display-only-failures below)

Took a TDD approach - once all failures resolved below, considered it good enough:

npm install -g @stoplight/spectral-cli
echo 'extends: ["spectral:oas"]' > .spectral.yaml
spectral lint openapi-v3.yaml --display-only-failures --ruleset .spectral.yaml