travis build matrix

[rvagg]

FYI I've updated the travis config and now have a built matrix for various versions of Node. Currently the latest 0.8, the last 5 versions of 0.5 and 0.11.4, 0.11.5 and 0.11.6. unfortunately 0.11.7 and 0.11.8 come with a consistently buggy npm that causes the builds to fail so I've removed them for now. Perhaps 0.11.9 will sort that out. We'll have to manually add new versions in as they are released.