Fix rspec on ruby 2.7

[hammerhead]

Summary of the changes / Why this is an improvement

Not much progress on upgrading itself, but at least makes the build green again. It failed before on the last ruby 2 version:

 activerecord-crate-adapter/ [main*] bundle exec rspec                      
Traceback (most recent call last):
	21: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/rspec-core-2.99.2/lib/rspec/core/runner.rb:17:in `block in autorun'
	20: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/rspec-core-2.99.2/lib/rspec/core/runner.rb:103:in `run'
	19: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/rspec-core-2.99.2/lib/rspec/core/command_line.rb:18:in `run'
	18: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/rspec-core-2.99.2/lib/rspec/core/configuration.rb:1065:in `load_spec_files'
	17: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/rspec-core-2.99.2/lib/rspec/core/configuration.rb:1065:in `each'
	16: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/rspec-core-2.99.2/lib/rspec/core/configuration.rb:1065:in `block in load_spec_files'
	15: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/rspec-core-2.99.2/lib/rspec/core/configuration.rb:1065:in `load'
	14: from /Users/niklasschmidtmer/Documents/GitHub/activerecord-crate-adapter/spec/activerecord/connection_adapters/crate/table_definition_spec.rb:22:in `<top (required)>'
	13: from /Users/niklasschmidtmer/Documents/GitHub/activerecord-crate-adapter/spec/activerecord/connection_adapters/crate/table_definition_spec.rb:22:in `require_relative'
	12: from /Users/niklasschmidtmer/Documents/GitHub/activerecord-crate-adapter/spec/spec_helper.rb:24:in `<top (required)>'
	11: from /Users/niklasschmidtmer/Documents/GitHub/activerecord-crate-adapter/spec/spec_helper.rb:24:in `require'
	10: from /Users/niklasschmidtmer/Documents/GitHub/activerecord-crate-adapter/lib/activerecord-crate-adapter.rb:22:in `<top (required)>'
	 9: from /Users/niklasschmidtmer/Documents/GitHub/activerecord-crate-adapter/lib/activerecord-crate-adapter.rb:22:in `require'
	 8: from /Users/niklasschmidtmer/Documents/GitHub/activerecord-crate-adapter/lib/active_record/connection_adapters/crate_adapter.rb:23:in `<top (required)>'
	 7: from /Users/niklasschmidtmer/Documents/GitHub/activerecord-crate-adapter/lib/active_record/connection_adapters/crate_adapter.rb:23:in `require'
	 6: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/activerecord-4.2.11.3/lib/active_record/base.rb:16:in `<top (required)>'
	 5: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/activesupport-4.2.11.3/lib/active_support/dependencies.rb:274:in `require'
	 4: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/activesupport-4.2.11.3/lib/active_support/dependencies.rb:240:in `load_dependency'
	 3: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/activesupport-4.2.11.3/lib/active_support/dependencies.rb:274:in `block in require'
	 2: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/activesupport-4.2.11.3/lib/active_support/dependencies.rb:274:in `require'
	 1: from /Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/activesupport-4.2.11.3/lib/active_support/core_ext/object/duplicable.rb:106:in `<top (required)>'
/Users/niklasschmidtmer/.rbenv/versions/2.7.8/lib/ruby/gems/2.7.0/gems/activesupport-4.2.11.3/lib/active_support/core_ext/object/duplicable.rb:111:in `<class:BigDecimal>': undefined method `new' for BigDecimal:Class (NoMethodError)

Checklist

• Link to issue this PR refers to (if applicable):
• CLA is signed

[hammerhead]

The usage of BigDecimal.new within activerecord itself currently blocks further Ruby version upgrades. As per that link above, Ruby 2.7 is the latest version still supporting BigDecimal.new.

[amotl]

Thank you very much!