Update gem spec

[Refs #4957544]
Merge pull request #4 from Thinkei/retrieve-employee-fields-4957544

Retrieve payroll employee fields
Ignore key file
Create Payroll API example
Ensure method call is identified as payroll
Merge branch 'master' of https://github.com/Thinkei/xero_gateway into retrieve-address-fields-49576891

* 'master' of https://github.com/Thinkei/xero_gateway:
  Ensure method call is identified as payroll
  Create Payroll API example
  Ignore key file
  Update gem spec
added home_address on get_payroll_employee_by_id