Update shoulda-matchers to 4.0.1

Fix the deprecation warning:

warning: BigDecimal.new is deprecated; use BigDecimal() method instead.

It was resolved in: https://github.com/thoughtbot/shoulda-matchers/commit/0c07bfb039b0fe9f9863e885ee3885e48f3b39d9

The 4.0.0 release changelog:

• Rails 5.x support
• Ruby 2.4+ support
• Improve negative versions of validation matchers
• Drop support for Ruby 2.0 and 2.1
• Drop support Rails 4.0 and 4.1