Use new demo GeoServer location

demo.opengeo.org currently redirects to demo.boundlessgeo.com, so this
change makes it so the new location is used directly.
Merge pull request #2997 from ahocevar/opengeo-boundless

Use new demo GeoServer location
Explain that return of ol.color.asArray should not be modified
Remove code that suggests that color arrays may be modified
Merge pull request #3000 from bartvde/color-docs

Explain that return of ol.color.asArray should not be modified