Check for several gem combinations

For #116 and #132.

`ProtectedAttributes` is defined only if you add the corresponding gem, not
if you use rails 3 without the gem. If you use rails 3 **and**
`strong_parameters`, or if you use rails 4, you have defined
`ActionController::StrongParameters`. If you use rails 4 **and**
`protected_attributes` you do have defined `ActionController::StrongParameters`,
but also `ProtectedAttributes` so it should short-circuit, right?

So I **think** these conditions satisfy every case. But I have no tests for
this. Although it should be possible to load different Gemfiles for each test
case, I don't know if it's worth the trouble..

What do you think?