Switch back to OkHttp 3

[musketyr]

Using the OkHttp 4.x forces users to include Kotlin libraries in the classpath which may have an unforeseen impact on the application. Usually dependencies error.

If there is nothing special you need from 4.x branch of OkHttp then please, consider downgrading the library to OkHttp 3.x and/or make the HTTP client switchable to a different implementation.