Bug 1021035 Adjust attribution contact page and update mapbox.js

Updates Mapbox to version v1.6.3, which includes a new default attribution for Open Street Maps.
Nope, that's now included in the main mapbox CSS: https://github.com/mozilla/bedrock/pull/2089/files#diff-7e4bf1577ec9028cc5bb9cc274fe26f3R818