Demo notebook: issue parsing signatures

[standage]

I'm getting a bunch of errors like the following when I execute cell 3 of demo/00-demo.ipynb.

loading SRR2060939_1.fastq.gz.sig
Error in parsing signature; quitting.
Exception: Unexpected symbol 'class' at 0

warning: no signatures loaded at given ksize/molecule type from SRR2060939_1.fastq.gz.sig

Probably these signature files were produced with an older version of sourmash and are now incompatible with the current parsing code?

[ctb]

demo/ directory removed in #631.