#4869 compatibility with older versions of apt

A fix for #4869.

This should preserve the current behaviour for modern low-level apt cache object while sticking to pre-https://github.com/ansible/ansible/commit/0d408ff29565d7c9814b5fc70ecea82db37c8e00 behaviour otherwise.

I could not find the tests for the module so I've just done fluent test on my boxes (including Lenny, Squeeze and Wheezy) to do some basic install/remove operations.

You might also want to check if regressions for https://github.com/ansible/ansible/issues/3421 are introduced — I don't know how to check that so I didn't do that.