update kraken database

[dpark01]

We've updated the Kraken database to include a few more taxa:

https://storage.googleapis.com/sabeti-public/meta_dbs/kraken_ercc_db_20160705.tar.lz4

Can you update the workflow to utilize this one?

[tomkinsc]

The lz4 database is not yet uploaded, but it will be soon!

[yifei-men]

@tomkinsc seems like it's up now, fetching 😄

[yifei-men]

Addressed by #30