test: add phpunit configuration file

Tests can now be run with just `vendor/bin/phpunit`.
test: update travis settings
doc: correct stated minimum PHP version
refactor: change service class to call toRequestXml method

Service and request object classes have to be refactored due to changes
in base classes.

[3d60a5e](https://github.com/davidtsadler/ebay-sdk/commit/3d60a5e29ff21edf971c1bcfea9efb96275b688b)
refactor: pass request object to callOperation method

The method parameters of BaseService::callOperation has changed so that
a request object must be passed is as the second parameter.

[27bff98](https://github.com/davidtsadler/ebay-sdk/commit/27bff98383695381e6872430ae91b495be9a9b4d)
doc: update CHANGELOG