[8u] RFR: JDK-8201553: Update FX build to use gradle 4.8

[Kevin Rushforth]

Phil & Arun,

Please review the following backport request to update the version of 
gradle used to build FX 8u to gradle 4.8:

https://bugs.openjdk.java.net/browse/JDK-8201553
http://cr.openjdk.java.net/~kcr/8201553/8u/webrev.00/

Details are in JBS.

-- Kevin