Update ol

https://github.com/openlayers/ol3/commit/25b99d5997182dd8a9fb11e7cf99589c9039a8cb
addPreRenderFunction renamed to beforeRender

https://github.com/openlayers/ol3/pull/972
Make ol.Attribution take an options object

https://github.com/openlayers/ol3/pull/947
Compliance with new ol.Overlay API

https://github.com/openlayers/ol3/pull/953
ol.Collection#getArray now exported by default