Allow Symfony3 (& v4) to be used with this package

[victormacko]

Currently the requirement for v2 of the framework-bundle prevents using Symfony v3 / v4 with this package ... could it be updated to be compatible with the newer versions of Symfony?

XeroBundle/composer.json

Line 16 in 93fb0d0

"symfony/framework-bundle": "2.*",

Awesome bundle btw!!