Detect Gradle from build.gradle.kts file

build-env

#1

Hi! With the advent of Gradle 5.0 you will have people using build.gradle.kts instead of build.gradle. The logic in https://github.com/travis-ci/travis-build/blob/master/lib/travis/build/script/shared/jvm.rb should account for that and not only check existence of build.gradle but also build.gradle.kts. The consequences of the existence of either should be the same, making this a quite straightforward and tame request.