Change to String, raise_error last argument accept String

http://apidock.com/ruby/Test/Unit/Assertions/assert_raise
If the last argument is a String, it will be used as the error message.
Merge branch 'fix/raise-error-argument' of https://github.com/sanemat/builder into sanemat

* 'fix/raise-error-argument' of https://github.com/sanemat/builder:
  Change to String, raise_error last argument accept String