Updated invoice_spec.rb

Changing this line will align the spec with the base requirements to be able to call .merchant_id on an invoice.

The ability to call .merchant on an invoice was not in the base requirements.
Merge pull request #12 from Diasporism/patch-1

Updated invoice_spec.rb
Change test to be order independent
Update gem summary
Update travis notifications
Add rake task to run student tests independently
Update spec names to not refer to specifics

This way we can change the data in the specs without changing the names.
Rework base expectations against new instructinos
Update extension specs per new requirements