Specify node version without patch version in `.travis.yml`

From http://docs.travis-ci.com/user/languages/javascript-with-nodejs/

0.10 is an alias for "the most recent 0.10.x release" and so on.
Please note that using exact versions (for example, 0.10.2) is highly
discouraged because as versions change, your .travis.yml will get
outdated and things will break.