fix version number

some packages, e.g. [pyramid_sockjs](https://pypi.python.org/pypi/pyramid_sockjs/0.3.9), already [depend](https://github.com/fafhrd91/pyramid_sockjs/blob/3db3071ed7e4e4e9013f82fb3cabff6756a3bf68/setup.py#L11) on `gevent >= 1.0rc2`, but according to `pkg_resources` version `1.0dev` doesn't meet that requirement:
```python
>>> from pkg_resources import parse_version
>>> parse_version('1.0dev') >= parse_version('1.0rc2')
False
```