Optimize Travis build matrix

[j4yk]

When #14 is merged, we will have three builds per push/PR, for the node versions 4, 5, and the latest. But all of them run the karma browser tests, which is nonesense. Ideally, we would have those three builds running the tests only for node (using mocha) and either

• one additional build that tests all browsers (using karma), or
• multiple additional builds that each run tests for exactly one browser

[j4yk]

Literature: https://docs.travis-ci.com/user/customizing-the-build/#Build-Matrix