Adding six as an explicit Python dependency on Travis.
Adding six as an explicit Python dependency on Travis.

See failed build:
https://travis-ci.org/dhermes/bossylobster-blog/builds/199118984

I can't to reproduce locally (i.e. html5lib has six as a dep,
so the error shouldn't occur).

dhermes/bossylobster-blog,dhermes/bossylobster-blog,dhermes/bossylobster-blog,dhermes/bossylobster-blog,dhermes/bossylobster-blog,dhermes/bossylobster-blog,dhermes/bossylobster-blog