upgrade to winston v3

[missinglink]

Some changes were made to Winston between versions 2 & 3.
https://github.com/winstonjs/winston/blob/master/UPGRADE-3.0.md

I think it would be beneficial to increase our test coverage in this repo to cover usage of the library by external Pelias modules.
Exporting winston in index.js adds complexity to that task, maybe we remove that export to isolate functionality in the future.

See https://github.com/winstonjs/winston/blob/master/UPGRADE-3.0.md

[orangejulius]

Oh yeah, I remember looking at this when #43 was opened.

Winston can do a lot!

We should probably make better use of it. I think it will take some time to learn but it would probably be well worth it for us to sit down, examine our usage of logging, learn Winston features, and develop some best practices.