Fix rust-client.channel.type

[AlexTugarev]

This PR fixes the type of the rust-client.channel config property.

null value leads to invalid schema. Ajv is failing to compile it:

schema is invalid: data.properties['rust-client.channel'].type should be equal to one of the allowed values, data.properties['rust-client.channel'].type[1] should be equal to one of the allowed values, data.properties['rust-client.channel'].type should match some schema in anyOf

[Xanewok]

Nice catch, thanks!

[AlexTugarev]

@Xanewok is there an ETA for 0.6.2 and would it include this one?

[Xanewok]

Released a 0.6.3 with the fixed package.json