python-Levenshtein>=0.11.1

This fixes the issue we worked around in
88e6cf1df064578c951c6ee8baec98e52f388884 a new release solves the problem.
Improve handling of escapes in wrapping

In some cases we could still split an escape which gettext won't do. An
escaped newline should keep with text on its left.
Travis: Install the development requirements

Previously used requirements was not providing pytest nor Sphinx.