Use string for Node version in .travis.yml

The node version is currently written as a number in `.travis.yml`, which is an issue as `0.10 === 0.1`. The version should be a string.

Fixes this [Travis build error](https://travis-ci.org/google/traceur-compiler/builds/31250700):

> $ nvm use 0.1
> N/A version is not installed yet