Add missing fileutils require in rubygems installer

Bundler inherits from rubygems installer and uses this method directly.
It can happen that `fileutils` is not yet required at that point.

To repro the issue I needed to make sure that the executables inside
dummy specs have the executable bit set. Otherwise a different code path
that _does_ require `fileutils` is run. This seems like a good thing
anyways since it better resembles the real world.

bundler/spec/install/gemfile/git_spec.rb

@@ -30,11 +30,15 @@
       expect(Dir["#{default_bundle_path}/cache/bundler/git/foo-1.0-*"]).to have_attributes :size => 1
     end
 
-    it "caches the git repo globally" do
+    it "caches the git repo globally and properly uses the cached repo on the next invocation" do
       simulate_new_machine
       bundle "config set global_gem_cache true"
       bundle :install
       expect(Dir["#{home}/.bundle/cache/git/foo-1.0-*"]).to have_attributes :size => 1
+
+      bundle "install --verbose"
+      expect(err).to be_empty
+      expect(out).to include("Using foo 1.0 from #{lib_path("foo")}")
     end
 
     it "caches the evaluated gemspec" do

bundler/spec/support/builders.rb

@@ -523,6 +523,7 @@ def @spec.validate(*); end
           file = Pathname.new(path).join(file)
           FileUtils.mkdir_p(file.dirname)
           File.open(file, "w") {|f| f.puts source }
+          File.chmod("+x", file) if @spec.executables.map {|exe| "#{@spec.bindir}/#{exe}" }.include?(file)
         end
         path
       end

lib/rubygems/installer.rb

@@ -530,6 +530,7 @@ def generate_plugins # :nodoc:
   def generate_bin_script(filename, bindir)
     bin_script_path = File.join bindir, formatted_program_filename(filename)
 
+    require 'fileutils'
     FileUtils.rm_f bin_script_path # prior install may have been --no-wrappers
 
     File.open bin_script_path, 'wb', 0755 do |file|