build.gradle cannot always parse gradle minor version

A gradle 4.1 snapshot build reports this version :
Gradle 4.1-20170718061434+0000

RC1 reports this
Gradle 4.1-rc-1

In both cases the FX build fails on a java exception with these because it
expects everything except for "." to be just decimals .. so it tries this
Integer.parseInt("1-rc-1")

I propose the somewhat more robust option of taking that string
and looking just for the leading digits.

def gradleMinor = Integer.parseInt(ver[1].split("[^0-9]")[0]);

This would fail only if there were none .. which doesn't seem to be a problem
in practice.

Whilst we don't actually use that minor version right now .. but it'll be important
when 4.1 is the minimum since 4.0.1 does NOT work.

However I do not intend to bump the required version yet.
Not until at least 4.1 is final but it is definite that to be able to use JDK 10
as the JDK to compile or test FX we will need 4.1 sometime.

It also is intended to officially support JDK 9 so we might bump this in a
JDK9 "updtes" forest too once both (4.1 GA and 9 updates) exist.

patch looks like this :
diff --git a/build.gradle b/build.gradle
--- a/build.gradle
+++ b/build.gradle
@@ -1073,7 +1073,7 @@
 if (gradle.gradleVersion != "3.1") {
     def ver = gradle.gradleVersion.split("[\\.]");
     def gradleMajor = Integer.parseInt(ver[0]);
- def gradleMinor = Integer.parseInt(ver[1]);
+ def gradleMinor = Integer.parseInt(ver[1].split("[^0-9]")[0]);
     def err = "";
     if (gradleMajor < 3) {
         err = "Gradle version too old: ${gradle.gradleVersion}; must be at least 3.0"