Revert "Detect HTTPS on rubygems.org which disallows HEAD"

This reverts commit da7b9d470cf713a32a6cbad4ba24562242fe1121.

Part of #506
Revert "Only attempt to upgrade rubygems.org to HTTPS"

This reverts commit eee98a3180dacaf32fa6a9d692291b37319305f8.

Part of #506
Fixup for @75a793e
Revert "Prefer HTTPS sources over HTTP sources"

This reverts commit 872ab85ddb1827c08bea4dac56bd1d4906951c83.

Fixes #506