Use latest OpenJDK 11 EA build as boot JDK

[kevinrushforth]

Modifies the Travis and Appveyor scripts to download and use the latest OpenJDK 11 EA build.

[kevinrushforth]

If we want to get this in prior to the GA of OpenJDK 11, then all that is needed is to change the build logic in appveyor.yml to grab the latest EA build referenced by the http://jdk.java.net/11/ page rather than hard-coding build 27 (also, I will revert my temporary change that bumped the minimum boot JDK).

[kevinrushforth]

This is now ready for review. Here are the relevant portions of the logs showing that the builds are now using JDK 11. Note that this does not change the minimum required to build FX. That will be done separately after JDK 11 is released.

Appveyor log (Windows) :

[00:02:41] JAVA_HOME: C:/jdk11
[00:02:41] JDK_HOME: C:/jdk11
[00:02:41] java.runtime.version: 11+28
[00:02:41] java version: 11
[00:02:41] java build number: 28
[00:02:41] jdk.runtime.version: 11+28
[00:02:41] jdk version: 11
[00:02:41] jdk build number: 28
[00:02:41] minimum jdk version: 10
[00:02:41] minimum jdk build number: 46

Travis log (Linux) :

JAVA_HOME: /home/travis/openjdk11
JDK_HOME: /home/travis/openjdk11
java.runtime.version: 11+28
java version: 11
java build number: 28
jdk.runtime.version: 11+28
jdk version: 11
jdk build number: 28
minimum jdk version: 10
minimum jdk build number: 46

Travis log (Mac) :

JAVA_HOME: /Users/travis/openjdk11
JDK_HOME: /Users/travis/openjdk11
java.runtime.version: 11+28
java version: 11
java build number: 28
jdk.runtime.version: 11+28
jdk version: 11
jdk build number: 28
minimum jdk version: 10
minimum jdk build number: 46

[kevinrushforth]

@brcolow @eugener Can you take a look?

[brcolow]

LGTM.

[eugener]

LGTM