fix series() to run in sequence

[haltcase]

Fixes #3

I'm not able to build or run tests since I'm on Windows (separate discussion - consider using cross-platform npm scripts eventually?) so you'll want to confirm. I did run a quick test script of my own though, which was this:

import { series } from './src'

let i = 0
const delayed = ms => {
  return new Promise(resolve =>
    setTimeout(() => (console.log(++i), resolve(i)), ms)
  )
}

series([
  () => delayed(1000),
  () => delayed(1000),
  () => delayed(1000),
  () => delayed(1000),
  () => delayed(1000)
]).then(res => {
  console.log('done', res)
})

You'll see the numbers 1-5 logged with about a second between each and then 'done' [1, 2, 3, 4, 5].

[haltcase]

After seeing the Travis test results I remarked this a WIP because I totally fixated on running the functions in series, and not so much as an ordered version of Promise.all. So this doesn't return the array of function results like it should.

Fixed it. This basically uses the solution mentioned by @TuckerWhitehouse but with a couple improvements:

1. drops resolve() usage entirely since it's actually not needed for series()
2. remove return await and just return the reduce() result - this is something I'd actually recommend across the whole package since using it adds an extra tick.

[haltcase]

@developit, you may not have had time to review but I'm just checking in - let me know if you have any thoughts on this.

[developit]

Apologies, I've been busy this week but I reviewed and it looks good.

[developit]

LGTM.