Install stringio for standalone spec.

- previously it was required already by net/http, but it is not anymore using ruby-trunk
- ruby/net-http@996d18a
- ruby/ruby@364044e#diff-a1d29a94def02829fd4f9ba591199acf079e028f5a2002a77c363eb01212e112

bundler/spec/install/gems/standalone_spec.rb

@@ -115,7 +115,7 @@
 
       realworld_system_gems "fiddle --version 1.0.6", "tsort --version 0.1.0"
 
-      necessary_system_gems = ["optparse --version 0.1.1", "psych --version 3.3.2", "yaml --version 0.1.1", "logger --version 1.4.3", "etc --version 1.2.0"]
+      necessary_system_gems = ["optparse --version 0.1.1", "psych --version 3.3.2", "yaml --version 0.1.1", "logger --version 1.4.3", "etc --version 1.2.0", "stringio --version 3.0.0"]
       necessary_system_gems += ["shellwords --version 0.1.0", "base64 --version 0.1.0", "resolv --version 0.2.1"] if Gem.rubygems_version < Gem::Version.new("3.3.3.a")
       realworld_system_gems(*necessary_system_gems, :path => scoped_gem_path(bundled_app("bundle")))