update compatibility to Java 9

[ehennum]

currently on 1.8

[georgeajit]

Running ./gradlew marklogic-client-api:clean
causes build failure.
Console output is as below:

* Where:
Build file '/space/linux64/HEAD/java-client-api/ml-development-tools/build.gradle' line: 6

* What went wrong:
Plugin [id: 'org.jetbrains.kotlin.jvm', version: '1.3.31'] was not found in any of the following sources:

- Gradle Core Plugins (plugin is not in 'org.gradle' namespace)
- Plugin Repositories (could not resolve plugin artifact 'org.jetbrains.kotlin.jvm:org.jetbrains.kotlin.jvm.gradle.plugin:1.3.31')
  Searched in the following repositories:
    Gradle Central Plugin Repository

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org