* lib/rubygems.rb, lib/rubygems/*, test/rubygems/*: Update rubygems-2.5.2.
It supports to enable frozen string literal and add `--norc` option for
disable to `.gemrc` configuration.
See 2.5.2 release notes for other fixes and enhancements.
https://github.com/rubygems/rubygems/blob/a8aa3bac723f045c52471c7b9328310a048561e0/History.txt#L3
* lib/rubygems/specification.rb: `coding` is affect only first line except
shebang.
* lib/rubygems/package.rb, lib/rubygems/package/*: ditto.
merge revision(s) 53707,53708,53709,53712,53777,53781: [Backport #12326]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@56003 b2dd03c8-39d4-4d8f-98ff-823fe69b080e