Require with extension for json files

[alexandrusavin]

Packaging apps (webpack etc.) default to js extensions.

[sindresorhus]

Sorry, I'm not going to change something because of Webpack. It's up to Webpack to support the Node.js ecosystem. Open an issue on Webpack ;)

[alextes]

Why not use read-pkg @sindresorhus?
I like reading JSON to be explicit, but that's just intuition.
Interestingly both read-pkg and load-json-file are yours. It's beside the point for this PR, but now I'm curious. Care to elaborate why we don't use them for got?

[sindresorhus]

@alextes require works perfectly for this use-case. We want to read the package.json once and cache it for the lifetime of the process. read-pkg would read it for every Got usage. require is also faster and less dependencies.