HEADS-UP: switching to gradle 2.8 for FX 9 builds

[Kevin Rushforth]

As a heads-up, we will be switching FX 9 production builds to using 
gradle 2.8 for building in the near future. See:

https://bugs.openjdk.java.net/browse/JDK-8090171

Very soon -- later this week if all goes well -- we will "flip the 
switch" to make 2.8 the default for building the FX jake bits (for 
Jigsaw). Not too long after that we will make 2.8 the default for 9-dev.

Here is a rough timeline that we are considering:

11/20 - switch 9-jake nightly builds to gradle 2.8
12/4 - switch 9-dev nightly builds to gradle 2.8
12/7 - switch 9 master builds to gradle 2.8
12/11 - push change to make 2.8 the minimum supported version

All developers will need to upgrade to gradle 2.8 before the 12/11 date. 
Note that you don't need to wait that long. I've been using gradle 2.8 
for over a week on all of my machines (and earlier gradle 2.X versions 
for longer).

Note that FX 8u will continue to build with gradle 1.8, although it is 
buildable with gradle 2.X for developers.

-- Kevin