switched :rubygems to 'https://rubygems.org' since :rubygems was deprecated

"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."

I changed it to 'https://rubygems.org' because it was recommended. Alternatively, 'http://rubygems.org' would preserve existing behavior and avoid the warning when running bundler, if OpenSSL being required is undesired.