Merge RubyGems 2.7.6 from upstream.
It fixed some security vulnerabilities.
http://blog.rubygems.org/2018/02/15/2.7.6-released.html
fix regexp literal warning.
* test/rubygems/test_gem_server.rb: eliminate duplicated character class warning.
[Bug #14481]
merge revision(s) 62422,62436: [Backport #14481]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@62443 b2dd03c8-39d4-4d8f-98ff-823fe69b080e