Investigate patches for recent Gatekeeper normalizations

[svenfuchs]

DO NOT MERGE

Only opening a PR here for potential discussion.

This is more of a quick experiment in order to figure out what would be
needed to get specs passing that are equivalent to the current
Gatekeeper specs.

I have marked all specs as pending that:

• exist in Gatekeeper
• fail here
• I don't think we need to support these cases any more

There is only one remaining case that I think we should continue
supporting:

travis-yaml only accepts env vars given as a Sequence:

  env:
    - FOO=foo
    - BAR=bar

While Gatekeeper also accepts them when given as a Map:

  env:
    FOO: foo
    BAR: bar

The same goes for nested global and matrix keys:

  env:
    global:
      - FOO=foo
      - BAR=bar
    matrix:
      - BAZ=baz
      - BUZ=buz

versus:

  env:
    global:
      FOO: foo
      BAR: bar
    matrix:
      BAZ: baz
      BUZ: buz