Added rm command to make sure gist_cacert.pem gets updated.

One of the recent updates to gist include a corrected certificate to use with the github website. The change can be pulled into homebrew by running `brew install --HEAD gist` (as there's no official release yet), however, the cached file `/tmp/gist_cacert.pem` doesn't get updated.

This adds an `rm` command to make sure we're always using the most recent certificate. Already asked the dev to implement a match between the file and code version. In the meantime, this should work as well.