Undo _JAVA_OPTIONS enrivonment variable

Ref sbt/sbt#3520

This undoes the environment variable addedin 2017-09-06 update of Travis CI:
https://docs.travis-ci.com/user/build-environment-updates/2017-09-06/

.travis.yml

@@ -1,5 +1,6 @@
-# Use Docker-based container (instead of OpenVZ)
 sudo: false
+dist: trusty
+group: stable
 
 cache:
   directories:
@@ -36,6 +37,10 @@ notifications:
   email:
     - sbt-dev-bot@googlegroups.com
 
+# Undo _JAVA_OPTIONS environment variable
+before_script:
+  - _JAVA_OPTIONS=
+
 script:
   # It doesn't need that much memory because compile and run are forked
   - sbt -J-XX:ReservedCodeCacheSize=128m -J-Xmx800M -J-Xms800M -J-server "$SBT_CMD"