HEADS-UP: switching to gradle 2.9 for FX 9 builds

[Kevin Rushforth]

Testing with gradle 2.9 looks good, so:  's/2.8/2.9/'

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

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

We will "flip the switch" to make 2.9 the default for building the FX 
jake bits (for Jigsaw) tomorrow. Not too long after that we will make 
2.9 the default for 9-dev.

Here is a rough timeline that we are planning:

11/20 - switch 9-jake nightly builds to gradle 2.9
12/1 - switch 9-dev nightly builds to gradle 2.9
12/7 - switch 9 master builds to gradle 2.9 (*)
12/14 - push change to make 2.9 the minimum supported version (*)

All developers will need to upgrade to gradle 2.9 before the 12/14 date, 
but you don't need to wait that long. I've been using gradle 2.X 
successfully on all of my machines for some time now.

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

(*) There is a possibility (unrelated to this) that we might skip the 
9-dev --> 9 master integration for the week of 12/7 which would delay 
the last two dates by one week; however, developers should not count on 
having the extra time

-- Kevin