Update upgrade.sh

[dropthemic]

Fix issue downloading packages with http being redirect to https

[dropthemic]

Was receiving error when running script:

[Upgrade] Downloading Cacti from http://www.cacti.net/downloads/cacti-latest.tar.gz

gzip: stdin: unexpected end of file
tar: Child returned status 1
tar: Error is not recoverable: exiting now

The website is redirecting to https.

[scline]

Nice catch 👍