Update and fix failing tests, identify Mocha globals, and cleanup readme

[Alhadis]

This PR is a combination of commits I prepared locally before an attempt to push revealed master is a protected branch. 😓 The changes are atomic and (hopefully) self-explanatory, but in short, they

1. Update the versions of Node and Mocha used to run the tests
2. Fix the currently-broken status badges in the readme
3. Identify Mocha's globals to js2-mode so functions like describe() and it() aren't flagged as errors
4. Improve the makefile's overall user-friendliness by:
   1. Only installing project dependencies when they don't exist
   2. Not assuming that an $NPM variable set in one's environment points to the npm(1) binary; I have $NPM set to /usr/local/lib/node_modules, so running make quickly failed.
      
      Note that it's still possible to override variables on a case-by-case basis like this:

      $ make NPM=/path/to/npm CASK=/path/to/cask install

I'll endeavour to keep future maintenance a little more review-friendly. 😉

/cc @scottaj

[Alhadis]

Nudging @scottaj for an update.