Support for Symfony 3 and Omnipay 3

XOL-4975 Support for Symfony 3 (#11)

* XOL-4975 Support for Symfony 3
* XOL-4975 update build matrix for travis
* Updated README

v1.2.1 Use default log format if custom format not specified

Merge pull request #10 from mogzol/master

Ensure default log value is set if not provided

v1.2 Configurable log format

Merge pull request #8 from anushr/log_format

Allow customization of log format

Configurable default gateway parameters

Merge pull request #3 from rushi/master

Combine parameters when initializing the gateway

Loggable requests and responses

Merge pull request #2 from kotrakrishna/master

Log all requests/response made through guzzle