Authentication fix for getting a REST gateway access token (#201)

V3 allow5.6 (#200)

* Allow 5.6

* Set lowest

* Remove listener

Require omnipay itself

Merge pull request #182 from islamdarwish/master

Implement list Plan and tests

Merge pull request #151 from pixelandtonic/master

Fixed a bug where the transaction reference returned on a completePayment response was not correct.

Merge pull request #150 from ryall/master

Support DateTime inputs as well as string. Add validation for require…

Merge pull request #121 from fraterblack/master

Implemented RestRefundCaptureRequest to refund captured payments. And RestVoidRequest to void a previously authorized payments

Merge pull request #142 from flo-sch/in-context-checkout

[In-Context] Create a new PayPal Express In-Context gateway extending…

Merge pull request #132 from rumeau/master

Fix invalid JSON error when parsing empty body rest response

Merge pull request #119 from Toolstation/toolstation

Implementing Express gateway order request