numpy deprecation warning with boundless read

[grovduck]

This one I think is pretty minor, but doing the below is giving a VisibleDeprecationWarning from numpy I think:

import rasterio
with rasterio.open('tests/data/RGB.byte.tif') as src:
    window = ((-10, 100), (-10, 100))
    data = src.read(1, window=window, boundless=True)

Output:

test.py:4: VisibleDeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
data = src.read(1, window=window, boundless=True)

It looks like it's happening when setting row and column offsets without rounding and casting to ints (ie. https://github.com/mapbox/rasterio/blob/master/rasterio/_io.pyx#L861-871). I didn't know when in the 'future' this was going to error rather than warn ... I'm using numpy 1.11.0.

[perrygeo]

@grovduck confirmed on numpy 1.11 and agree that that's the likely place to fix it. I'll try to work up a PR today.