Switch to latest stable version of Kotlin language dependency

[Cliabhach]

At some point last year, the Kotlin team deprecated org.jetbrains.kotlin:kotlin-stdlib-jre7 and recommended using org.jetbrains.kotlin:kotlin-stdlib-jdk7 instead. It's something to do with the security model of Java 9+'s JVM being more strict about what files are allowed to be in what archive (I think), but the code in the two artifacts is basically the same. So this project could define

'kotlin': '1.2.71'

and

org.jetbrains.kotlin:kotlin-stdlib-jdk7:${versions.kotlin}

without too much trouble. @jobinlawrance what do you think?

[jobinlawrance]

Sure, but I am really caught up with something else. I am open to a pull request.

[Cliabhach]

FYI I've tagged a release including this and two or so minor related changes at https://github.com/Cliabhach/okhttp3-downloadprogress-interceptor/releases/tag/1.0.2-a1 .