replace casperjs with selenium

did you mean ``os.environ.get`` here?
Actually, I was told no attribute `env` when:
```
>>> os.env.get('CI_BASE_URL', 'http:127.0.0.1:8000')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'env'
```