Replace `source :rubygems` with https url for rubygems.org in Gemfile

Builder 1.3.2 complained:
The source :rubygems is deprecated because HTTP requests are insecure.
Please change your source to 'https://rubygems.org' if possible, or 'http://rubygems.org' if not.

Caveat: I know nothing about Ruby; I simply followed the instructions, tested, and observed that `bunder install` no longer produces this warning.