Fix boxify_points to correctly use a negative buffer

[sgoodm]

The following line in the boxify_points utils function was creating a positive buffer value since the min of the cell size dimensions is already a negative number. This was resulting in a buffered pixel box that would overlap with surrounding cells when using all_touched.

buff = -0.01 * min(rast.affine.a, rast.affine.e)

[sgoodm]

Not sure what is going on with the checks failing. Runs fine locally

[perrygeo]

Thanks @sgoodm - the fix looks good to me.

The test failure is likely unrelated - due to #155 Rasterio 1.0 changes. I'm going to address those today, at which point I'll merge this and get out a new release.

[coveralls]

Coverage remained the same at 98.606% when pulling f2f5c27 on sgoodm:fix_boxify_points into 9eb57b7 on perrygeo:master.