Fix Elm package badge

[j-maas]

With the release of Elm 0.19.0, the package file changed (from elm-package.json to elm.json), which breaks the badge.

[shields-ci]

	Warnings
⚠️	This PR modified the server but none of the service tests. That's okay so long as it's refactoring existing code. Otherwise, please consider adding tests to the service: https://github.com/badges/shields/blob/master/doc/service-tests.md#readme

	Messages
📖	✨ Thanks for your contribution to Shields, @y0hy0h!

Generated by 🚫 dangerJS

[paulmelnikow]

Hi! Thanks for this fix.

#1958 will move this code out of server.js, and this will need to wait until that lands. At that time this change can be re-applied in services/elm-package/elm-package.service.js.

[paulmelnikow]

#1958 is merged.

@y0hy0h would you like to open a new PR applying this fix to services/elm-package/elm-package.service.js? Drop [elm-package] into the PR title and the tests will be run automatically.

It would be great to get the fix deployed ASAP!

[j-maas]

@paulmelnikow Done! Thanks for your guidance!