Use HTTPS when downloading the latest WordPress package

See #350

"libcurl performs peer SSL certificate verification by default"
http://curl.haxx.se/docs/sslcerts.html

This is better practice and eliminates the current redirect from
HTTP to HTTPS. We'll leave the `-L` flag, as future redirects are
possible.

Thanks @nacin
Merge pull request #452 from Varying-Vagrant-Vagrants/https-wp

Use HTTPS when downloading the latest WordPress package