Fix Bundler::GemRequireError in test environment

[cesar-tonnoir]

Running RAILS_ENV=test rake db:migrate in a Rails 5.1 project using jsonapi-resources-matchers in its :test gems group leads to the following error:

rake aborted!
Bundler::GemRequireError: There was an error while trying to load the gem 'jsonapi-resources-matchers'.
Gem Load Error is: undefined method `configure' for RSpec:Module
Backtrace for gem load error is:
/home/cesar/.rvm/gems/ruby-2.3.3/gems/jsonapi-resources-matchers-1.0.0/lib/jsonapi/resources/matchers/integrations/rspec.rb:2:in `<top (required)>'

This was due to the fact that in a standard Rails 5.1 project, rspec/core is not required by default before Bundler.require(*Rails.groups) runs in application.rb.

Requiring rspec/core prior to the inclusion of the matchers fixes the issue.