Use the latest stable Node.js version on Travis CI and enable caching

[timvandermeij]

This patch instructs Travis CI to:

• Use the latest version of Node.js at all times. This makes it possible for us to catch any breakage in newer Node.js versions soon and allows us to make use of the newest functionality of Node.js. Note that the version does not need to be in quotes when it is a string, which is now the case. Previously, Travis CI had an issue with their YAML parser, causing for example 0.10 to be interpreted as 0.1, hence the need for the quotes back then (see Fails to detect Node 0.10 in .travis.yml travis-ci/travis-ci#977).
• Cache the node_modules directory. This prevents downloading and installing the same packages for each build and thus speeds up the build quite a bit (I have seen improvements in build times of 20+ seconds). To make sure that new packages are installed when we add them to package.json, we have npm install, and to ensure that packages are updated when the version changes in package.json we have npm update. I have tested that this works as expected. Finally we move the gulp-cli installation to the install section as that it where it belongs.

Supersedes #7500.

[timvandermeij]

Trivial.