Switched broken pypip.in badges to shields.io

[movermeyer]

Hello, this is an auto-generated Pull Request. (Feedback?)

Some time ago, pypip.in shut down. This broke the badges for a bunch of repositories, including pymdptoolbox. Thankfully, an equivalent service is run by shields.io. This pull request changes the badges to use shields.io instead.

Unfortunately, PyPI has removed download statistics from their API, which means that even the shields.io "download count" badges are broken (they display "no longer available". See this). So those badges should really be removed entirely. Since this is an automated process (and trying to automatically remove the badges from READMEs can be tricky), this pull request just replaces the URL with the shields.io syntax.

[coveralls]

Coverage remained the same at 86.831% when pulling 389b2f42d62489f013352a3b3eed2c0ede6c0af9 on movermeyer:fix_badges into 7c96789cc80e280437005c12065cf70266c11636 on sawcordwell:master.