Fix python setup: avoid import numpy in setup.py

Currently `pip install xgboost` will raise traceback like this

```
Traceback (most recent call last):
  File "<string>", line 20, in <module>
  File "/tmp/pip-build-IAdqYE/xgboost/setup.py", line 20, in <module>
    import xgboost
  File "./xgboost/__init__.py", line 8, in <module>
    from .core import DMatrix, Booster
  File "./xgboost/core.py", line 12, in <module>
    import numpy as np
ImportError: No module named numpy
```

We should avoid importing numpy in setup.py and let pip install numpy and scipy automatically.
That's what `install_requires` for.

python-package/xgboost/VERSION

@@ -0,0 +1 @@
+0.4