Run tests against more versions of node on travis CI

[neocotic]

We're currently only testing against node v0.8. Not only is this a very old version, but we need to test against more versions. I really just want to concentrate on the last version of previous major releases and the latest versions of node. For example:

• 0.8
• 0.10
• 0.12
• 4.0
• 4.2
• 5.0

I'm not keen on adding "iojs" since that's now merged into node so shouldn't really be required.

[neocotic]

Since I've since decided (in the past hour 😄) that only newer versions of Node are going to be supported, we'll only need to concentrate on the following versions:

• 4.0
• 4.2
• 5.0

[neocotic]

Marking this as invalid as the Europa 🌑 rewrite will involve testing against a broader range of Node.js versions.