Support build.gradle.kts

Gradle 5.0 (and earlier versions as preview) allows Gradle scripts in Kotlin. They have .kts suffix (build.gradle.kts, settings.gradle.kts) and thus are totally overlooked by fdroidserver (in scanner.py checkupdates.py, build.py, etc.).

This is a show-stopper for inclusion of apps that use Gradle scripts in Kotlin.