Fix branch-whitelist logic

[asmeurer]

When --branch-whitelist is passed with no arguments, (i.e., it's empty), it
should force the whitelist to be empty. Only when it is not passed at all
should it default to {master}.

I haven't tested this yet.

Should fix #329.

[asmeurer]

Looks like the conda cryptography module is broken in Python 3.6.

[asmeurer]

Several old builds such as 1cecd14 were pushing from the empty --branch-whitelist build.

I pushed a commit to this branch that should force a deploy to verify that it doesn't do that anymore.

[asmeurer]

Now it doesn't push https://travis-ci.org/drdoctr/doctr/jobs/460406133#L3272. You can also see the tag- directory wasn't updated on gh-pages.