Change --gh-pages-docs default

[asmeurer]

It's a backwards incompatible change, but --gh-pages-docs defaulting to docs is not great. . is probably what people expect. Maybe we should make it a required argument.

CC @ocefpaf

[gforsyth]

I think it's reasonable to change it to .

[gforsyth]

We could make it a required argument and make the helper text to be pasted into the .travis.yml --gh-pages-docs . as the default?

[ocefpaf]

I do not know the reasons to deploy to docs, and I am fine with #128 (comment) solution, but maybe the project is still young and a backwards incompatible change is not that bad...

[asmeurer]

If it's required it really ought to be the argument to doctr deploy, like doctr deploy ..

[gforsyth]

Done in #157.