[MRG] fix astype usage to prevent copying

Removed were possible; using copy=False (from utils.fixes) were needed.

Also some C integer type fixes to gradient boosting.

Fixes #2206.
This causes a breakage as higlighted by travis:

```
File "_gradient_boosting.pyx", line 291, in sklearn.ensemble._gradient_boosting._random_sample_mask (sklearn/ensemble/_gradient_boosting.c:3899)

ValueError: Does not understand character buffer dtype format string ('?')
```