Ruby 1.9 compat: Avoid using the return value of FileUtils.mkdir_p, as it does...

Ruby 1.9 compat: Avoid using the return value of FileUtils.mkdir_p, as it does not return a String but an Array in Ruby 1.9

[#2018 state:committed milestone:2.3.5]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

railties/lib/commands/plugin.rb

@@ -232,7 +232,7 @@ def install_using_http(options = {})
     
     def install_using_git(options = {})
       root = rails_env.root
-      install_path = mkdir_p "#{root}/vendor/plugins/#{name}"
+      mkdir_p(install_path = "#{root}/vendor/plugins/#{name}")
       Dir.chdir install_path do
         init_cmd = "git init"
         init_cmd += " -q" if options[:quiet] and not $verbose