Rasterio: Switch github default merge operation to merge and squash

[brendan-ward]

GitHub recently introduced a new feature that allows the merge button to also squash commits: https://github.com/blog/2141-squash-your-commits

I propose we adopt the first option: Allow merge commits and commit squashing

In practice, we seem to be doing this some of the time anyway, once our PRs are approved but prior to merging. This would make it much easier to be consistent, and clear out a bit of the clutter from trying to get around Travis build issues...

This needs to be done by @sgillies if we want to go this route.

[perrygeo]

+0 Personal preference is to avoid squashing; I think there's value in the commit history. Commit messages can contain information about past design decisions and provide clues to potential bugs. But having the _option_ to squash is fine by me, as long as we use it sparingly and wisely (i.e. don't squash away useful history)

[sgillies]

@perrygeo let's try allowing for the sprint and re-evaluate afterward.

[sgillies]

Nevermind, squashes are already allowed.

[digitalmaster]

Is there a way to make "Squash and Merge" the default PR merge option in the last for an entire project?