runtests: add option to only keep images of failed image tests

the imgur upload script seems to get rejected after too many uploads,
and in most cases only the failed images are of interest.
Merge remote-tracking branch 'upstream/releases'

Conflicts:
	.travis.yml
travis: fix matplotlib version to 1.3.1
Merge remote-tracking branch 'upstream/releases'

Conflicts:
	.travis.yml
travis: install basemap==1.0.7 from wheelhouse
coverage should ignore scripts directories