Fix #69 "bob.io.image is a test dependency but it is imported"

bob/bio/base/script/preprocess.py

@@ -12,7 +12,10 @@ import numpy
 
 import bob.core
 import bob.io.base
-import bob.io.image
+try:
+  import bob.io.image
+except ImportError:
+  pass
 
 
 def command_line_arguments(command_line_parameters):

test-requirements.txt

@@ -2,3 +2,4 @@ bob.db.atnt
 bob.io.image
 matplotlib
 gridtk
+nose