Adding support and infra for Node 4, 6, and 8.

[jnwng]

utilizing CircleCI's workflows feature, we can make sure that we can safely establish the engines field here.

i had explored compiling this code using babel's new TypeScript preset and using babel-preset-env to ensure we were transpiling everything we needed, but i hit issues both with making sure sourcemaps were covering the original TypeScript source code as well as the fact that our use of instance methods like Array.prototype.includes couldn't safely be polyfilled without polluting the global scope. do the simple thing here and just use lodash and hope the test harness catches any further regressions.

i dont have access to the release flow, so whoever is reviewing this should ensure that my semantic-release changes are okay. thank you!