Build failure on current gradle

[nsoft]

Fresh checkout of the project building against gradle 3.1 had the following error

FAILURE: Build failed with an exception.

* What went wrong:
A problem was found with the configuration of task ':startScripts'.
> No value has been specified for property 'mainClassName'.

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

BUILD FAILED

This appears to be due to a change since gradle 2.3 (we were on 2.1 until recently I think)... see discussion @ spring-boot where they ran into this change as well. spring-projects/spring-boot#2679