Make JUnit5Runner usable with Early Access builds (#194)

Fixes this crash:
```
Exception in thread "main" java.lang.NumberFormatException: For input string: "21-ea"
	at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)
	at java.base/java.lang.Integer.parseInt(Integer.java:661)
	at java.base/java.lang.Integer.parseInt(Integer.java:777)
	at com.github.bazel_contrib.contrib_rules_jvm.junit5.JUnit5Runner.getSystemExitToggle(JUnit5Runner.java:57)
	at com.github.bazel_contrib.contrib_rules_jvm.junit5.JUnit5Runner.main(JUnit5Runner.java:24)
```

java/src/com/github/bazel_contrib/contrib_rules_jvm/junit5/JUnit5Runner.java

@@ -53,7 +53,8 @@ public static void main(String[] args) {
   private static SystemExitToggle getSystemExitToggle() {
     // In Java 8 and lower, the first part of the version is a 1.
     // In Java 9 and higher, the first part of the version is the feature version.
-    int featureVersion = Integer.parseInt(System.getProperty("java.version").split("\\.")[0]);
+    // Major versions of early Access builds have an "-ea" suffix.
+    int featureVersion = Integer.parseInt(System.getProperty("java.version").split("[.-]")[0]);
     if (featureVersion == 1) {
       featureVersion = 8;
     }