Refactor NestedFundingRoundSerializer
Refactor serializer unit tests
Fix CategorySerializer test
Fix rubocop complaint
Ignore Investment#investor_guid

It's used in a `map(&:investor_guid)`, but rails_best_practices doesn't
catch that.
Remove company_with_category factory
Use build_stubbed
Use build_stubbed
Remove unneeded tap

`build_stubbed` creates a company with a category_id
Merge pull request #81 from cloudspace/serializer_specs

Serializer specs