Feature/Host auditor serializer

Ability to use Truemail::Auditor instance represented as json directly:

Truemail.host_audit.as_json

=>
# Serialized Truemail::Auditor instance
{
  "date": "2020-08-31 22:33:43 +0300",
  "current_host_ip": "127.0.0.1",
  "warnings": {
    "dns": "A-record of verifier domain not refers to current host ip address", "ptr": "PTR-record does not reference to current verifier domain"
  },
 "configuration": {
    "validation_type_by_domain": null,
    "whitelist_validation": false,
    "whitelisted_domains": null,
    "blacklisted_domains": null,
    "not_rfc_mx_lookup_flow": false,
    "smtp_safe_check": false,
    "email_pattern": "default gem value",
    "smtp_error_body_pattern": "default gem value"
  }
}

1. Updated Truemail::Auditor, Truemail::Validator
2. Refactored Truemail::Log::Serializer namespaces
3. Implemented Truemail::Log::Serializer::AuditorJson
4. Optimized regex
5. Updated gem dependencies
6. Updated linters configs
7. Updated gem changelog
8. Updated gem version to 1.9.0