Enable allow_unicode when dumping bootcamp cache

I think this should fix #503.  It seems that by default PyYAML wants to output non-ASCII characters using escape sequences rather than writing a non-ASCII byte stream.  This will allow it to write the file out containing non-ASCII characters without escaping them (by default it writes UTF-8).

The escape sequences it was using previously are perfectly valid for YAML but it's possible that some tools elsewhere in the pipeline weren't recognizing it properly.  I think having just plain UTF-8 end-to-end should avoid the problem, I hope.