Update test harness for Bundler 1.16 compatibility

This commit fixes Travis failures due to the recent introduction of
Bundler 1.16.0. Both are related to the new style of binstubs that are
generated by 1.16.0.

First, add `bundler` to the list of binstubs that we explicitly generate
when creating the test Rails app. The `bin/bundle` binstub that ships as
part of `rails new` is in the pre-1.16 format, and conflicts with the
1.16 format binstubs that are generated for rake and rspec. Fix this by
replacing the default `bin/bundle` with a fresh one generated by
`bundle binstubs bundler --force`.

Second, the `Bundle.with_clean_env` wrapper seems to remove some
important information from the environment that causes binstubs to fail
with errors like `uninitialized constant Gem (NameError)`. Fix this by
using `Bundle.with_original_env` instead (which is what the Bundler
documentation recommends). Because Travis also sets its own
`BUNDLER_GEMFILE` variable, I had to also reset that one manually.

Author: mattbrictson

Rakefile

@@ -65,7 +65,7 @@ namespace :generate do
       in_example_app do
         sh "./travis_retry_bundle_install.sh 2>&1"
         # Rails 4 cannot use a `rails` binstub generated by Bundler
-        sh "bundle binstubs rspec-core rake --force"
+        sh "bundle binstubs bundler rspec-core rake --force"
         sh "bundle binstubs railties" unless File.exist?("bin/rails")
 
         application_file = File.read("config/application.rb")
@@ -93,7 +93,8 @@ def in_example_app(*command_opts)
     app_dir = opts.fetch(:app_dir, app_dir)
   end
   Dir.chdir(app_dir) do
-    Bundler.with_clean_env do
+    Bundler.with_original_env do
+      ENV["BUNDLE_GEMFILE"] = "Gemfile"
       sh *command_opts unless command_opts.empty?
       yield if block_given?
     end
@@ -187,7 +188,7 @@ namespace :no_active_record do
         in_example_app(:app_dir => example_app_dir) do
           sh "./travis_retry_bundle_install.sh 2>&1"
           # Rails 4 cannot use a `rails` binstub generated by Bundler
-          sh "bundle binstubs rspec-core rake --force"
+          sh "bundle binstubs bundler rspec-core rake --force"
           sh "bundle binstubs railties" unless File.exist?("bin/rails")
 
           application_file = File.read("config/application.rb")