Fix the unit tests by adding missing parameter

Without the fix, here is the test command and error:

nosetests -v -s tests

======================================================================
ERROR: test_libthumbor.test_usage
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/Users/fdu/Documents/work/fork/libthumbor/tests/test_libthumbor.py", line 39, in test_usage
    image=image
TypeError: encrypt() takes exactly 17 arguments (16 given)

----------------------------------------------------------------------