Use https for remotes by default

I was testing this script on git.ruby-lang.org to use its git version,
but it did not work because the server's default user doesn't have SSH
keys.

https works for everyone, so it's a safer default. You shouldn't need to
push to that remote from ruby/ruby anyway.

tool/sync_default_gems.rb

@@ -492,7 +492,7 @@ module SyncDefaultGems
     # Fetch the repository to be synchronized
     IO.popen(%W"git remote") do |f|
       unless f.read.split.include?(gem)
-        `git remote add #{gem} git@github.com:#{repo}.git`
+        `git remote add #{gem} https://github.com/#{repo}.git`
       end
     end
     system(*%W"git fetch --no-tags #{gem}")