Remove unsupported node-4 build from Travis CI Builds - all builds failing due to node-4 build issue

[julianfresco]

I have seen that Pull Requests are all failing due to the node-4 build having an issue with yarn:
The command "eval yarn " failed

Example failure:
https://travis-ci.org/makotot/react-scrollspy/builds/499932228?utm_source=github_status&utm_medium=notification

Since nodejs.org doesn't support node 4 as of last year, it should not be used for Travis CI. Notice that in the example, all other builds are succeeding.