Updating InstallCasper function in caspercheck.sh

Updating the InstallCasper function in caspercheck.sh. I had been using
"-d" for the file test operator, as bundle-style QuickAdd packages are
detectable as directory. However, valid flat QuickAdd packages fail the
check, since they are xar archive files.

To fix this issue, I'm updating the file test operator to use "-e" to
test and see if the QuickAdd package exists without having to specify
if it's a file or directory. This should accommodate both use cases.