Retain Plone KGS compatibility by pinning oauth2client and six dependencies

The recipe can no longer be installed in the same buildout configuration as a Plone since because the compatible version of ``six`` is conflicting: Plone requires an older version as our dependency ``oauth2client``.

Therefore we pin down ``oauth2client`` as well as ``six`` to a compatible version for both.

Error was:
```
The constraint, 1.2.0, is not consistent with the requirement, 'six>=1.6.1'.
While:
  Installing.
  Getting section i18n-build.
  Initializing section i18n-build.
  Installing recipe ftw.recipe.translations.
Error: Bad constraint 1.2.0 six>=1.6.1
```