* lib/rubygems: Update to RubyGems 2.4.1 master(713ab65)

Complete history at:
  https://github.com/rubygems/rubygems/blob/master/History.txt#L3-L216
* test/rubygems:  ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47582 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/core_ext/kernel_gem.rb

@@ -26,6 +26,11 @@ module Kernel
   # Kernel#gem should be called *before* any require statements (otherwise
   # RubyGems may load a conflicting library version).
   #
+  # Kernel#gem only loads prerelease versions when prerelease +requirements+
+  # are given:
+  #
+  #   gem 'rake', '>= 1.1.a', '< 2'
+  #
   # In older RubyGems versions, the environment variable GEM_SKIP could be
   # used to skip activation of specified gems, for example to test out changes
   # that haven't been installed yet.  Now RubyGems defers to -I and the
@@ -51,7 +56,9 @@ module Kernel
     end
 
     spec = Gem::Dependency.new(gem_name, *requirements).to_spec
-    spec.activate if spec
+    Gem::LOADED_SPECS_MUTEX.synchronize {
+      spec.activate
+    } if spec
   end
 
   private :gem