Fix bad path manipulation in macOS VM
Fix bad path manipulation in macOS VM

We should have been using `stringByAppendingPathComponent:`, not
`stringByAppendingPathExtension`. This didn't matter until Sierra, where
they improved/increased error checking.

@bpollack #39
timfel/squeakvm,OpenSmalltalk/vm,peteruhnak/pharo-vm,bencoman/pharo-vm,bencoman/pharo-vm,timfel/squeakvm,peteruhnak/pharo-vm,OpenSmalltalk/vm,OpenSmalltalk/vm,bencoman/pharo-vm,timfel/squeakvm,OpenSmalltalk/vm,bencoman/pharo-vm,peteruhnak/pharo-vm,OpenSmalltalk/vm,timfel/squeakvm,peteruhnak/pharo-vm,peteruhnak/pharo-vm,bencoman/pharo-vm,peteruhnak/pharo-vm,peteruhnak/pharo-vm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,bencoman/pharo-vm,bencoman/pharo-vm,peteruhnak/pharo-vm,timfel/squeakvm,timfel/squeakvm,bencoman/pharo-vm,OpenSmalltalk/vm,bencoman/pharo-vm