Update package.json engines field to allow modern node versions

[emjimenez]

The package throws incompatibility errors with yarn and node v10.4.0:

$ node -v
v10.4.0
$ yarn add ln-service
yarn add v1.7.0
[1/4] 🔍  Resolving packages...
[2/4] 🚚  Fetching packages...
error ln-service@21.0.2: The engine "node" is incompatible with this module. Expected version "8.12.0".
error Found incompatible module
info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this command.

I installed the package with "yarn add --ignore-engines ln-service", but but it would be better to update the engines field in package.json.

[alexbosworth]

It's not tested with 10.4.0 though

[alexbosworth]

I'll just remove the engine field