Java 9 is not compatible with Gradle 2.13

[oleg-sidorov]

I tried to build the project in "complete" folder using IntelliJ IDEA + JDK 9 + Gradle and encountered the error:

"Could not determine java version from '9.0.4 '"

Then I upgraded Gradle to version 4.4.1 in gradle-wrapper.properties:

distributionUrl=https://services.gradle.org/distributions/gradle-4.4.1-bin.zip

and it helped.

Please update Gradle in the source code.

[Greg L. Turnquist]

@jbsf Since we've updated the guide to Gradle 4.6, perhaps you would like to check again.