Fix specs, update CI, and fix Atom Beta

[Arcanemagus]

glslangValidator seems to have recently updated the output for one of the warnings used in the specs. This updates the specs accordingly.

I also took the opportunity to update the CI configuration:

• Remove macOS testing: This has become unbearably slow lately and virtually never showed anything different from the other OS builds
• Fix a few issues with the CircleCI config
  • glslangValidator now caches, while still building the latest version.
  • Move to using the atom/ci script

Fixes #53.

[Arcanemagus]

Hmmmm, now it seems the AppVeyor builds are using an out of date version...

[Arcanemagus]

Of course, https://github.com/KhronosGroup/glslang/commit/7a44a31dee9ff0b65cda876c87e508ff87bc3503 broke their master build, which in turn breaks the specs here...

[Arcanemagus]

53 should be fixed in d0c6567, bringing it in here so we can get a fully green build on master.