Describe the bug
Slic3r is not able to connect to Octoprint using HTTPS

To Reproduce
Steps to reproduce the behavior:

1. Go to Printer Settings and enter the Hostname and API key of an octoprint server
2. Click on "test"
3. See error:

I wasn't able to connect to OctoPrint (501 Protocol scheme 'https' is not supported (LWP::Protocol::https not installed)). Check hostname and OctoPrint version (at least 1.1.0 is required).

Expected behavior
Slic3r should be able to use HTTPS to communicate with Octoprint

Desktop (please complete the following information):

• OS: macOS 10.14.4
• Version 1.3.1-dev-85dd663