Update LSNSURLSessionHook.m

In the current implementation, the presence of [NSURLSessionConfiguration class] in the ternary appeared to cause the class to be loaded - even though it isn't returned - and that was causing side effects in iOS7.  Replacing that with NSClassFromString worked.  I also split that long line up.
Merge pull request #76 from augustj/iOS8-Fix

Update LSNSURLSessionHook.m