release for stable coroutine support

[Nikky]

At the moment no version of kgraphql works with non-experimental coroutines
as of now i am unable to update to ktor 1.0.0-beta-1

with 0.3.0-alpha
it does not compile:

Class 'com.github.pgutkowski.kgraphql.schema.Schema' is compiled by a pre-release version of Kotlin and cannot be loaded by this version of the compiler

with 0.2.8 it breaks at runtime
kotlinx/coroutines/experimental/CommonPool does not exist

i will attempt to make a PR

[Paweł Gutkowski]

Thank you for your PR @NikkyAI, I've released new version 0.3.0-beta with your contribution and fixes from master branch.