MAINT-2155, DRTVWR-278: Make Mac updater honor app bundle name in .dmg.

If you run "/Applications/Second Life Beta Materials.app", and the version
manager directs you to update to a .dmg containing a release candidate whose
app bundle name is "Second Life Viewer.app", update_install.py used to copy
the contents of "Second Life Viewer.app" into "/Applications/Second Life Beta
Materials.app". Changed it so that if the application bundle name differs, we
leave the running app bundle alone, installing instead to the app bundle name
from the .dmg.