encode stdout messages as ascii

fixes the following error when outputting filenames that contain unicode characters

Traceback (most recent call last):
  File "manage.py", line 18, in <module>
    execute_from_command_line(sys.argv)
  File "/app/.heroku/python/lib/python2.7/site-packages/django/core/management/__init__.py", line 399, in execute_from_command_line
    utility.execute()
  File "/app/.heroku/python/lib/python2.7/site-packages/django/core/management/__init__.py", line 392, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/app/.heroku/python/lib/python2.7/site-packages/django/core/management/base.py", line 242, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/app/.heroku/python/lib/python2.7/site-packages/django/core/management/base.py", line 285, in execute
    output = self.handle(*args, **options)
  File "/app/.heroku/python/lib/python2.7/site-packages/imagekit/management/commands/generateimages.py", line 26, in handle
    self.stdout.write('  %s\n' % file)
  File "/app/.heroku/python/lib/python2.7/site-packages/django/core/management/base.py", line 65, in write
    if ending and not msg.endswith(ending):
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 15: ordinal not in range(128)