Inconsistency between Travis and local deploy

[hroncok]

On Travis:

• gh-pages is not pulled and a single commit is force pushed, the branch will have only one commit

Locally:

• depending on your local state, the commit will be added on the top of gh-pages branch and force pushed

We can do one of the following:

• fetch and reset before we commit (resulting in a history of deployments in gh-pages)
• always remove the branch before committing (resulting in a single commit in gh-pages, consistent with README)

[encukou]

Or we can make it configurable.

I'd opt for keeping single commit, or defaulting to that.

[hroncok]

I've decided making it configurable would add an unnecessary complexity for almost no reason. Went with the documented behavior.

[hroncok]

Deleting the local branch is not enough. ghp-import checks origin/gh-pages and rebases on top of that https://github.com/davisp/ghp-import/blob/3fd376d66d1626da00c623193ff04169afb9c97f/ghp_import.py#L69

[hroncok]

It would need to do something like rm .git/refs/remotes/origin/gh-pages if origin is the right remote.