Updated code for Omnipay 3

[emergingdzns]

I modified the composer file and readme to reference my repo for my own testing and development (global find/replace). Feel free to ignore those changes. The only changes in the composer.json you need to keep are the require and require dev omnipay version numbers.

In the readme you have the instruction to require version 1.0. I changed it to 2.0.

Otherwise very little needed to be changed. Omnipay 3 no longer uses Guzzle so the 404 error check is unnecessary in the AbstractRequest sendData method.

Also the send data is now $this->httpClient->request('POST',$this->getEndpoint(), [], http_build_query($data)); instead of the old ->post() method due to the change in client library.

In the phpunit xml file the Mockery listener is removed per the Omnipay upgrade documentation.

To be honest, I have no idea how to run the unit tests nor how to edit the tests to adjust to the changes (if they even need to be changed).

[emergingdzns]

I have no idea how to do the travis testing. I had a couple changes I had to make today for refunds.

[zburke]

Log doesn't appear to be used anywhere. Can it be omitted here?

[emergingdzns]

yes sorry. i have since removed it. I forgot to do another PR. I made another couple changes. I'll do a PR again.

[emergingdzns]

Ok so I think that does it. The latest change was to the refund request. The problem is that BP has so many different APIs I was mixing up the formatting. They want people to be using their bp10emu endpoint now instead of the bp20post which is what we are using now. However it's not scheduled for deprecation yet so it's ok to use for now.