Ignore differences in the bitrate of ffmpeg-generated MP3s

The information returned by libmagic for MP3 files includes
a bitrate, but it's misleading since the files are VBR
(variable bit rate) rather than CBR.  In any case, the bitrate
of the files isn't an important thing to test for the
functionality of the site, so ignore that part of the
returned file info.
Compare the length of generated audio files less precisely

With different versions of ffmpeg, there seem to be slight
differences in lengths of the generated files (although
they sound identical).  I haven't investigated this in
detail, but my hypothesis is that this is to do with frame
padding in the MP3, so relax the strictness of the test to
greater than the length of a frame in one of these files.
Use ffmpeg decoding directly, and compare within 0.2s.
Add logging in as part of test set up, so tests pass.
Merge branch 'relax-audio-comparison-tests'

Conflicts:
	speeches/tests/audio_helper_tests.py