Changes from the previous release: ([diff](https://github.com/alphatwirl/alphatwirl/compare/v0.18.6...v0.18.7))
- changed behavior of `RoundLog`
- returns the underflow bin for `0` and negative values if `min`
is given. (previously, returns `0` for `0` and `None` for a
negative number even when `min` is given)
- returns the overflow bin for `inf` if `max` is given.
(previously, returns `None` for `inf` even when `max` is given)
- optimized `RoundLog` and `Round` for speed, e.g., replacing linear
search with binary search
- updated zenodo badge, using all-versions badge
- organized requirements [\35](https://github.com/alphatwirl/alphatwirl/issues/35)
- requirements/doc.txt for sphinx doc, used at Read the Docs
- requirements/test.txt for test, used at travis-ci
- added CHANGELOG.md