Pin NPM version to 1.4.0 for travis tests, 2.0.0-beta seems to be breaking

We use a `before_install` to update Node 0.8's NPM version to a version that supports the `^` version identifier, which is now standard. Travis seems to be updating NPM to 2.0.0-beta, which appears to be breaking during the tests. Pin the version to 1.4.0 as a fix.