Also adds travis-before_install-append.yml which will be used by PR to scaffold-package-command to allow for package-specific stuff to be added to .travis.yml.

Waiting for that matching PR before merging this one.

The 81a803b commit moves the remove_old_images() function into needs_regeneration(), and splits the $image_sizes check in two so that thumbnails don't get destroyed if an image is skipped.

This non-destructiveness should also really extend to the no metadata case, but haven't come up with a nice way to do this, so leaving for the moment.

Also added some tests of this behaviour, and added some extra comments.

The 8f7bc79 commit reinstates the caching of the ImageMagick build, adding another Travis append file.

I'll do the PR on scaffold-package-command for adding the extra Travis stuff very shortly...