Change `requirements.txt`

After a conversation about `requirements.txt` in issue #218 and
almost 20 days with no more comments I'm using @ahmadia suggestion.

Note that the `requirements.txt` not cover the packages need for
create a badge, because of that I'm also updating the
`README-badge.md`.
Merge pull request #240 from r-gaia-cs/issue218

Change `requirements.txt`