Setting JRUBY_OPTS without cext.enabled or --2.0

https://travis-ci.org/rspec/rspec-support/jobs/45770207#L544

See travis-ci/travis-ci#3067

Per http://docs.travis-ci.com/user/ci-environment/
Travis sets JRUBY_OPTS="--server -Dcext.enabled=false -Xcompile.invokedynamic=false"
but current JRuby throws warnings when passed -Dcext.enabled

Until that is fixed, it is sensible to just set
JRUBY_OPTS="--server -Xcompile.invokedynamic=false"

It's common to modify JRUBY_OPTS in travis projects, and in fact
script/functions.sh modimodimodimodifies it!
  # idea taken from: http://blog.headius.com/2010/03/jruby-startup-time-tips.html
  export JRUBY_OPTS="${JRUBY_OPTS} -X-C" # disable JIT since these processes are so short lived

Use -Xcompat.version=2.0 per `jruby --properties` in place of
`--2.0`, which  wasn't being respected

Author: bf4

.travis.yml

@@ -22,19 +22,24 @@ rvm:
   - 2.2
   - ruby-head
   - ree
-  - jruby-18mode
-  - jruby
-  - jruby-head
   - rbx
 matrix:
   include:
     - rvm: jruby
-      env: JRUBY_OPTS='--2.0'
-  allow_failures:
+      env: JRUBY_OPTS='--server -Xcompile.invokedynamic=false -Xcompat.version=2.0'
     - rvm: jruby-head
+      env: JRUBY_OPTS='--server -Xcompile.invokedynamic=false'
+    # These two are temporary until https://github.com/travis-ci/travis-ci/issues/3067 is solved.
+    - rvm: jruby-18mode
+      env: JRUBY_OPTS='--server -Xcompile.invokedynamic=false'
+    - rvm: jruby
+      env: JRUBY_OPTS='--server -Xcompile.invokedynamic=false'
+  allow_failures:
     - rvm: ruby-head
     - rvm: rbx
     # These two are temporary until https://github.com/travis-ci/travis-ci/issues/3067 is solved.
     - rvm: jruby-18mode
+      env: JRUBY_OPTS='--server -Xcompile.invokedynamic=false'
     - rvm: jruby
+      env: JRUBY_OPTS='--server -Xcompile.invokedynamic=false'
   fast_finish: true