ubuntu默认pip安装mkdocs后缺失python-markdown-math

ubuntu16.04利用virtualenv全新安装mkdocs后
```
pip install mkdocs
# 在源码目录里执行，mkdocs serve，会提示ImportError: Failed loading extension 'mdx_math' from 'mdx_math', 'markdown.extensions.mdx_math' or 'mdx_mdx_math'
```
还需要用pip手动额外安装，方可本地运行
```
sudo pip install python-markdown-math
```

更新，默认apt安装也会报类似的错误