.ico is binary too

Solves:

error occurred trying to convert text to unicode in .well-known/favicon.ico
Traceback (most recent call last):
  File "/usr/local/bin/dexy", line 8, in <module>
  [...]
  File "/Library/Python/2.7/site-packages/dexy-0.5.7-py2.7.egg/dexy/artifact.py", line 569, in output_text