Sync Installation class with API spec (#3135)

Adds the following attributes:
- access_tokens_url
- account
- app_slug
- contact_email
- created_at
- events
- has_multiple_single_files
- html_url
- permissions
- repositories_url
- repository_selection
- single_file_name
- single_file_paths
- suspended_at
- suspended_by
- updated_at