Partially restore hack

lib/bundler/version.rb

@@ -3,6 +3,24 @@
 module Bundler
   VERSION = "2.1.0.pre.1".freeze
 
+  if ENV["BUNDLE_SPEC_RUN"]
+    # We're doing this because we might write tests that deal
+    # with other versions of bundler and we are unsure how to
+    # handle this better.
+    def self.overwrite_loaded_gem_version
+      begin
+        require "rubygems"
+      rescue LoadError
+        return
+      end
+      return unless bundler_spec = Gem.loaded_specs["bundler"]
+      return if bundler_spec.version == VERSION
+      bundler_spec.version = Bundler::VERSION
+    end
+    private_class_method :overwrite_loaded_gem_version
+    overwrite_loaded_gem_version
+  end
+
   def self.bundler_major_version
     @bundler_major_version ||= VERSION.split(".").first.to_i
   end