Merge branch 'ab/fix-poison-tests' into maint

Update tests to pass under GETTEXT_POISON (a mechanism to ensure that output strings that should not be translated are not translated by mistake), and tell TravisCI to run them. * ab/fix-poison-tests: travis-ci: add job to run tests with GETTEXT_POISON travis-ci: setup "prove cache" in "script" step tests: fix tests broken under GETTEXT_POISON=YesPlease