manually set install requires to 0.10.dev so that install works when web...

...assets 0.10.dev is installed

Attempting to install Flask-Assets currently fails with 

```
Could not find a version that satisfies the requirement webassets==dev (from Flask-Assets==0.10.dev) (from versions: 0.4, 0.5, 0.6, 0.7.1, 0.7, 0.8, 0.9)
No distributions matching the version for webassets==dev (from Flask-Assets==0.10.dev)
```
You will still need to upload `webassets==0.10.dev` to PyPi or add to the Flask-Assets documentation that `webassets` should be install directly from GitHub.