Update class attribute with attachInterrupt
You've run into a classic trap.  There are a couple of things to think about.  First off, if this is a singleton class (as in there will only ever be one instance) then you can just have a non-member function that you use with attachInterrupt and have that function just call the member function you want.  Normally what I would do would be to create the instance in the .cpp and put an extern declaration for it in the header so that it can just be used without creating it.  Look at how Serial works for an example.  Then put all the configuration type stuff into a begin or init method to call from setup.  That way the non-member function for the interrupt can be part of the header and the user never has to see or mess with it and it feels like it's part of the class.  

If this is not a singleton class and you might one day have several instances of this in one program then you have to start asking yourself how will the interrupt know which instance to act upon.