Bump to omnipay-common 3.0 dependency

[roland-jungwirth]

I tried updating to the Omnipay 3.0 but this breaks due to a dependency of this package:

  Problem 1
    - Installation request for omnipay/common ^3.0@dev -> satisfiable by omnipay/common[3.0.x-dev, v3.0-RC1, v3.0-RC2, v3.0-alpha.1, v3.0-alpha.2, v3.0-alpha.3, v3.0-alpha.4, v3.0-beta.1, v3.0-beta.2, v3.0-beta.3, v3.0.0, v3.0.1, v3.0.2].
    - omnipay/payfast 2.0.x-dev requires omnipay/common ~2.0 -> satisfiable by omnipay/common[2.5.x-dev].
    - omnipay/payfast v2.0.0 requires omnipay/common ~2.0 -> satisfiable by omnipay/common[2.5.x-dev].
    - omnipay/payfast v2.1.0 requires omnipay/common ~2.0 -> satisfiable by omnipay/common[2.5.x-dev].
    - omnipay/payfast v2.1.1 requires omnipay/common ~2.0 -> satisfiable by omnipay/common[2.5.x-dev].
    - omnipay/payfast v2.1.2 requires omnipay/common ~2.0 -> satisfiable by omnipay/common[2.5.x-dev].
    - omnipay/payfast v2.1.3 requires omnipay/common ~2.0 -> satisfiable by omnipay/common[2.5.x-dev].
    - omnipay/payfast v2.2 requires omnipay/common ~2.0 -> satisfiable by omnipay/common[2.5.x-dev].
    - Conclusion: don't install omnipay/common 2.5.x-dev
    - Installation request for omnipay/payfast ~2.0 -> satisfiable by omnipay/payfast[2.0.x-dev, v2.0.0, v2.1.0, v2.1.1, v2.1.2, v2.1.3, v2.2].

Are you planning on updating the dependency requirement of this package to omnipay/common 3.0 soon?

[barryvdh]

If you want to make a PR, sure.