GitHub review for JDK-8201553: Update FX build to use gradle 4.8

[Kevin Rushforth]

Hi Phil & Johan,

I have initiated a pull request on GitHub as a review for:
JBS: https://bugs.openjdk.java.net/browse/JDK-8201553
PR: https://github.com/javafxports/openjdk-jfx/pull/92

It is now ready to be reviewed, and once the review is done, merged into 
the develop branch of the GitHub sandbox. I'm doing it there first to 
ensure that the CI builds pass with gradle 4.8 (which they do, as can be 
seen from the PR).

-- Kevin