fix: use ruby < 2.3 syntax. (#108)

v1.6.1 added code that uses ruby's safe navigation operator which was
only available for ruby versions >= 2.3. Since this gem has ruby
requirement of >= 2.0, this will create issues for users using ruby <
2.3, such as users using version 1.88.84 of
pact-foundation/pact-ruby-standalone which ships with travelling ruby
version 2.2.