[Travis] Make autoload load `Xcodeproj::PlistHelper`.

lib/cocoapods/generator/acknowledgements/plist.rb

@@ -6,7 +6,7 @@ module Pod
       end
 
       def save_as(path)
-        Xcodeproj.write_plist(plist, path)
+        Xcodeproj::PlistHelper.write(plist, path)
       end
 
       def plist

spec/unit/generator/acknowledgements/plist_spec.rb

@@ -54,7 +54,7 @@ describe Pod::Generator::Plist do
     basepath = config.sandbox.root + 'Pods-acknowledgements'
     given_path = @generator.class.path_from_basepath(basepath)
     expected_path = config.sandbox.root + 'Pods-acknowledgements.plist'
-    Xcodeproj.expects(:write_plist).with(equals(@generator.plist), equals(expected_path))
+    Xcodeproj::PlistHelper.expects(:write).with(equals(@generator.plist), equals(expected_path))
     @generator.save_as(given_path)
   end
 end