sympy/sympy — pull request #8719 by aktech (2014-12-31 12:32:03+00:00)
Title: Add Positive assumption helper for gamma function, Fixes #8524 

The Positive assumption helper for gamma function is missing:
So the the following code returns None:

```python
>>> n = Symbol('n', real=True, positive=True)
>>> gamma(n).is_positive
```
With this PR, Positive assumption works well for gamma function.

Fixes #8524