fix #1148 disable npm spinner on travis

npm 1.4.11+ uses spinner (progress indicator) which messes with the output
of the test suite, making it less readable. There's no need for it during
Travis builds.

(you can see the output with spinner enabled e.g. [here](https://travis-ci.org/ariatemplates/ariatemplates/builds/26922151))
While this syntax seems to work, there's no mention of the equal operator in the documentation of [npm-config](https://www.npmjs.org/doc/cli/npm-config.html), and there's no added value to it. Myself I wouldn't bother with it.