add JRuby 9.2 and Java 11 to the ci mix

The `JAVA_OPTS` environment variable is used by JRuby to launch the JVM with
appropriate options; by adding specific `--add-opens` flags, we are able to
enable the specific reflective access that is necessary to run in Javas >= 9.

.travis.yml

@@ -36,6 +36,10 @@ matrix:
     - rvm: jruby-9.1.9.0
     - rvm: ruby-head
     - env: "CHILDPROCESS_POSIX_SPAWN=true"
+  include:
+  - rvm: jruby-9.2.5.0
+    jvm: openjdk11
+    env: "JAVA_OPTS='--add-opens java.base/java.io=org.jruby.dist --add-opens java.base/sun.nio.ch=org.jruby.dist'"
   exclude:
     # Travis does not provide 1.9.3 on OSX
   - rvm: 1.9.3

README.md

@@ -173,6 +173,8 @@ ChildProcess.logger = logger
 ## Caveats
 
 * With JRuby on Unix, modifying `ENV["PATH"]` before using childprocess could lead to 'Command not found' errors, since JRuby is unable to modify the environment used for PATH searches in `java.lang.ProcessBuilder`. This can be avoided by setting `ChildProcess.posix_spawn = true`.
+* With JRuby on Java >= 9, the JVM may need to be configured to allow JRuby to access neccessary implementations; this can be done by adding `--add-opens java.base/java.io=org.jruby.dist` and `--add-opens java.base/sun.nio.ch=org.jruby.dist` to the `JAVA_OPTS` environment variable that is used by JRuby when launching the JVM.
+
 
 # Implementation