Update Travis CI build to use OpenJDK 11

[joschi]

Currently, the builds on Travis CI are failing because the build environment is trying to use Java 8 to compile Everest (which requires Java 11 according to the POM).

This PR updates the Travis CI build configuration to:

• Use OpenJDK 11
• Use the default build steps for Maven-based Java projects on Travis CI
• Use a bundled version of Maven 3.6.0 instead of whatever version of Maven is installed
• Use the Ubuntu Xenial build environment

[RohitAwate]

Thanks Jochen! I had been postponing this for a while. ;)

I'll merge this in as it is, but I was wondering if you could help me out with one more thing.
No pressure, of course.

I can't figure out why the build doesn't run with the Exec Maven Plugin. I've been having the exact same issue without Maven Wrapper. Due to this, I need to build Everest with the JavaFX SDK. I've talked about it here.

This is the stack trace I get:

java.lang.ExceptionInInitializerError
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
    at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke (Method.java:566)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:297)
    at java.lang.Thread.run (Thread.java:834)
Caused by: java.lang.RuntimeException: Internal graphics not initialized yet
    at com.sun.glass.ui.Screen.getScreens (Screen.java:70)
    at com.sun.javafx.tk.quantum.QuantumToolkit.getScreens (QuantumToolkit.java:709)
    at com.sun.javafx.tk.quantum.QuantumToolkit.getMaxRenderScale (QuantumToolkit.java:736)
    at com.sun.javafx.tk.quantum.QuantumToolkit.loadImage (QuantumToolkit.java:745)
    at javafx.scene.image.Image.loadImage (Image.java:1052)
    at javafx.scene.image.Image.initialize (Image.java:802)
    at javafx.scene.image.Image.<init> (Image.java:618)
    at com.rohitawate.everest.Main.<clinit> (Main.java:32)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
    at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke (Method.java:566)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:297)
    at java.lang.Thread.run (Thread.java:834)

Thanks again! :)

[joschi]

@RohitAwate See the latest commit, which fixed the error when running ./mvnw exec:java.

[joschi]

See https://github.com/warmuuh/milkman for a more full-featured Java-based alternative for Postman.