Stop trying to access NSMutableArray

Unfortunately the private structure of NSMutableArray changes
considerably between versions of iOS. (see http://git.io/CAbScg)

I tried to fix this by using ksobjc_getIvarValue, because the
field names have stayed constant throughout, but unfortunately
NSMutableArray uses bit-packing for flags which is not supported
by ksobjc_getIvarValue (or Apple's object_getInstanceVariable
for that matter).

For now I think the safest thing to do is to not dig into mutable
arrays.
Merge pull request #73 from ConradIrwin/fix-nsmutablearray

Stop trying to access NSMutableArray