## Summary
`BigDecimal.new` is deprecated since BigDecimal 1.3.3 for Ruby 2.5.
This PR suppresses the following warnings.
```console
% ruby -v
ruby 2.6.0dev (2018-09-06 trunk 64648) [x86_64-darwin17]
% RUBYOPT=-w bundle exec rake
(snip)
/Users/koic/src/github.com/Shopify/liquid/lib/liquid/utils.rb:49:
warning: BigDecimal.new is deprecated; use Kernel.BigDecimal method
instead.
/Users/koic/src/github.com/Shopify/liquid/lib/liquid/utils.rb:53:
warning: BigDecimal.new is deprecated; use Kernel.BigDecimal method
instead.
```
## Other Information
The following is a change of BigDecimal 1.3.3 for Ruby 2.5 related to this PR.
- https://github.com/ruby/bigdecimal/commit/533737338db915b00dc7168c3602e4b462b23503
- https://github.com/ruby/bigdecimal/commit/16738ad0aca158d3c70123990d8caa5789818177
