(PUP-3346) Don't allow insecure SSL connections w/curl

The appdmg and pkgdmg providers were calling curl with -k (—-insecure)
which allows curl'ing over insecure HTTPS connections. I can't see why
this would ever be desired, and connections often fail attempting
to establish a connection.

This commit removes the insecure option.
(PUP-3346) Add test to ensure -k flag is not used

To ensure there are no regressions, update the tests so that they
expect curl to be called without the "-k" flag.
Merge pull request #3124 from seanknox/fix/master/pup-3346-fix-insecure-curl-options

(PUP-3346) Don't allow insecure SSL connections w/curl