Always use Gem::Installer mutex

If Gem::Installer is subclassed (as in Bundler) then @install_lock will
not be initialized and Bundler will not be able to install gems.

Using an explicit class allows subclassing to work transparently.

See
d96e0d5#commitcomment-7729421

lib/rubygems/installer.rb

@@ -258,7 +258,7 @@ def install
 
     say spec.post_install_message unless spec.post_install_message.nil?
 
-    self.class.install_lock.synchronize { Gem::Specification.add_spec spec }
+    Gem::Installer.install_lock.synchronize { Gem::Specification.add_spec spec }
 
     run_post_install_hooks