Use BigDecimal for multiple_of check

Fixes overflow error exposed by: json-schema-org/JSON-Schema-Test-Suite#438

```
FloatDomainError: Infinity
    /Users/dharsha/repos/json_schemer/lib/json_schemer/schema/base.rb:367:in `floor'
    /Users/dharsha/repos/json_schemer/lib/json_schemer/schema/base.rb:367:in `validate_numeric'
    /Users/dharsha/repos/json_schemer/lib/json_schemer/schema/base.rb:378:in `validate_number'
    /Users/dharsha/repos/json_schemer/lib/json_schemer/schema/base.rb:277:in `validate_type'
    /Users/dharsha/repos/json_schemer/lib/json_schemer/schema/base.rb:198:in `validate_instance'
```

Author: davishmcclurg

lib/json_schemer.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 require 'base64'
+require 'bigdecimal'
 require 'ipaddr'
 require 'json'
 require 'net/http'

lib/json_schemer/schema/base.rb

@@ -355,8 +355,7 @@ def validate_numeric(instance, &block)
         validate_exclusive_minimum(instance, exclusive_minimum, minimum, &block) if exclusive_minimum
 
         if multiple_of
-          quotient = data / multiple_of.to_f
-          yield error(instance, 'multipleOf') unless quotient.floor == quotient
+          yield error(instance, 'multipleOf') unless BigDecimal(data.to_s).modulo(multiple_of).zero?
         end
       end