According to the Node LTS schedule, v4 is no longer maintained, and v6 is in maintenance mode until April 2019.

We shouldn't bother supported unsupported node versions. [email protected] (our next release) should be the last minor release to support v4.

• bump package.engines
• change README ("ShellJS is proudly tested on every node release since...")
• drop v4 & v5 from travis/appveyor
• do all the same for shx