upgrade fs-extra and remove fs-promise

[Jonahss]

When installing, npm warns
npm WARN deprecated fs-promise@2.0.3: Use mz or fs-extra^3.0 with Promise Support

fs-extra was actually already a dependency. I updated it and removed fs-promise.

require('./lib/files.js').test().then(console.log)
// true

[davesag]

I'd love to see this PR get merged as those deprecation warnings are annoying. Thanks in advance

[Jonahss]

I checked that the libraries are compatible, so it should be good to go.

[benjamincburns]

@MaiaVictor - anything blocking getting this merged & released? This package is included as a transitive dependency of web3.js (web3-bzz, specifically), a very popular tool in the ethereum ecosystem. As a result, anyone doing ethereum development today is encountering deprecation warnings when they install packages with this dependency.

I'm happy to help out if anything needs doing before this can be merged, unless of course @Jonahss handles it first!