Remove requirements badge

This displays that things are insecure even when it is only in the test
environment and gives a misleading appearance that the library is
insecure. Currently it complains because our tests use pyyaml and we
utilize the feature to pass arbitrary code via yaml to peform tests.
This is a controlled environment, and not a risk as it is only done for
testing, and you'd have to alter the tests to inject bad code. If you
can do that, you could just change the actual code.

README.md

@@ -1,7 +1,6 @@
 [![Unix Build Status][travis-image]][travis-link]
 [![Windows Build Status][appveyor-image]][appveyor-link]
 [![Coverage Status][codecov-image]][codecov-link]
-[![Requirements Status][requires-image]][requires-link]
 [![pypi-version][pypi-image]][pypi-link]
 ![License][license-image-mit]
 
@@ -23,8 +22,6 @@ License is MIT except for a few exceptions.  See [LICENSE](https://github.com/fa
 [travis-link]: https://travis-ci.org/facelessuser/pymdown-extensions
 [appveyor-image]: https://img.shields.io/appveyor/ci/facelessuser/pymdown-extensions/master.svg?label=Windows%20Build&logo=appveyor
 [appveyor-link]: https://ci.appveyor.com/project/facelessuser/pymdown-extensions
-[requires-image]: https://img.shields.io/requires/github/facelessuser/pymdown-extensions/master.svg
-[requires-link]: https://requires.io/github/facelessuser/pymdown-extensions/requirements/?branch=master
 [pypi-image]: https://img.shields.io/pypi/v/pymdown-extensions.svg?logo=python&logoColor=white
 [pypi-link]: https://pypi.python.org/pypi/pymdown-extensions
 [license-image-mit]: https://img.shields.io/badge/license-MIT-blue.svg