Arrow functions, let/const, and cleanup

[JustinBeckwith]

This introduces let/const instead of var, moves to arrow functions in some cases, removes explicit 'use strict', and cleans up a few imports.

[codecov-io]

Codecov Report

Merging #732 into master will decrease coverage by 0.08%.
The diff coverage is 94.91%.

@@            Coverage Diff             @@
##           master     #732      +/-   ##
==========================================
- Coverage   79.58%   79.49%   -0.09%     
==========================================
  Files           8        6       -2     
  Lines         240      239       -1     
  Branches       53       53              
==========================================
- Hits          191      190       -1     
  Misses         30       30              
  Partials       19       19

Impacted Files	Coverage Δ
lib/utils.ts	100% <ø> (ø)	⬆️
lib/generator_utils.ts	60% <ø> (ø)	⬆️
lib/apirequest.ts	97.33% <100%> (+0.03%)	⬆️
lib/googleauth.ts	100% <100%> (ø)
lib/googleapis.ts	78.18% <100%> (ø)	⬆️
lib/discovery.ts	67.64% <89.65%> (ø)	⬆️

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[jmdobry]

Do you plan on switching to ES2015 classes, destructuring, etc. in a separate PR?

[JustinBeckwith]

@jmdobry Yeah, a basic outline of my plan is in #503. After this, next step is es6 style imports and classes.