chore(repo): remove use of npx + update eslint

[peter-mouland]

Update dependencies after spotted a comment about babel-eslint in this previous pr

https://github.com/postcss/postcss-color-hex-alpha/pull/15

One question is around the post css peeDependency, would you prefer to bump that too?

[gazaret]

Hi! Any updates?

[peter-mouland]

HI @jonathantneal

I see some of this work has now been completed elsewhere, so I've rebased and removed npx (since it's not needed in this use-case) and updated eslint