use '.trasnsient' method to specify transient properties

4a4bc9b3155669c2e4c9f24246a3cc99f413d8a8 was wrong. `#transient?` should
expect a string, not symbol (See #14). However, I just found out `transient`
method which is defined in `Jenkins::Model' module that `Jenkins::Tasks::BuildWrapper`
includes, and I felt using this method is more Ruby-way.