Update ol3 vector-api, using new functionality

This PR does the following:

- Adapt Makefile to assure multiple `make updateol` work in succession for vector-api branch of ol3
- Updates ol3 to https://github.com/openlayers/ol3/commit/057cda42be7f6b27dcb9106dd00870570bcef139
- Using clear() method on vector source to remove all features.