Remove unnecessary line in MANIFEST.in for docs/avatar.png.

 * The buildbot for ooni-backend was showing:
   Downloading/unpacking txtorcon>=0.7 (from -r .requirements.travis (line 21))
   Downloading txtorcon-0.7.tar.gz (128kB): 128kB downloaded
   Running setup.py egg_info for package txtorcon
   warning: no files found matching 'docs/avatar.png'
Fixed unflushed buffers in tests, fixed ident-comparison on string for PyPy
Merge pull request #51 from lukaslueg/master

Fixed two tests, fixes for PyP