Use Guzzle7 as default implementation (thephpleague#615)

* Use Guzzle7 adapter as default implementation

Allow guzzle6 v2 adapter (thephpleague#556)

Merge pull request thephpleague#502 from thephpleague/v3-5.6

Allow 5.6 and 7.0

Update composer.json

Tweak composer/travis

Merge pull request thephpleague#221 from dercoder/master

ecoPayz Gateway added

Bump

Bump omnipay/common requirement

Update omnipay/common to v2.1

Test that package installs successfully