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/__init__.py

@@ -5,12 +5,16 @@ Contributors: https://github.com/dmlc/xgboost/blob/master/CONTRIBUTORS.md
 """
 
 from __future__ import absolute_import
+
+import os
+
 from .core import DMatrix, Booster
 from .training import train, cv
 from .sklearn import XGBModel, XGBClassifier, XGBRegressor
 from .plotting import plot_importance, plot_tree, to_graphviz
 
-__version__ = '0.4'
+VERSION_FILE = os.path.join(os.path.dirname(__file__), 'VERSION')
+__version__ = open(VERSION_FILE).read().strip()
 
 __all__ = ['DMatrix', 'Booster',
            'train', 'cv',