Add jruby-head to Travis config; allow it to fail.
Change psp_reference field limit

So that test notifications made from the Adyen website can work.

Currently, when testing notification Adyen sents this param:

   "pspReference" => "testnotification_NOTIFICATIONTEST"

Which unfortunately does exceed the previous 30 chars limit.
Merge pull request #55 from pjg/fix_adyen_test_notifications

Make test notifications sent from Adyen website work
Add `default_api_params` requirement to docs

Closes #41
Merge pull request #56 from pjg/doc_updates

Add `default_api_params` requirement to docs
Fix API #references call with empty recurring contracts

Previously, if you had no recurring contrancts and you executed a
`references` method to list IDs of those contracts, you would get this
error:

    NoMethodError: undefined method `map' for nil:NilClass`

This fixes that, so you'll get an empty list instead.
Merge pull request #57 from pjg/api_fix

Fix API #references call with empty recurring contracts
Released adyen gem version 1.3.2.