Deprecate Gem::ConfigMap in favor of RbConfig

I'm not sure what it was ever for.  It just copies RbConfig::CONFIG
entries, so let's just use the original source now.
Use RbConfig ruby_version for Gem.ruby_api_version

By default this has the same value as MAJOR.MINOR.TEENY, but can be
overriden from ruby's ./configure --with-ruby-version= which makes
RubyGems more flexible for packagers.

Fixes #770