Fix non integer zoom request when using pinch zoom

Pinch zooming on mobile phones generates invalid non integer zoom levels. The Google server reports error 404 and the tiles disappear during zoom. Rounding the zoom level fixes this issue.

Similar problem to this: https://github.com/Leaflet/Leaflet/issues/1934
Merge pull request #125 from nhinze/patch-3

Fix non integer zoom request when using pinch zoom