Unset _JAVA_OPTIONS env variable on Travis to suppress warning from JVM

> Picked up _JAVA_OPTIONS: -Xmx2048m -Xms512m

* https://travis-ci.org/rspec/rspec-core/jobs/274481617
* https://travis-ci.org/rspec/rspec-expectations/jobs/275700473
* https://travis-ci.org/rspec/rspec-mocks/jobs/275700697
* https://travis-ci.org/rspec/rspec-support/jobs/275700691

Author: yujinakayama

travis/.travis.yml

@@ -5,6 +5,7 @@ cache:
   directories:
     - ../bundle
 before_install:
+  - unset _JAVA_OPTIONS
   - "script/clone_all_rspec_repos"
   # Note this doesn't work on JRUBY 2.0.0 mode so we don't do it, the excluded versions are broken on Ruby 2.3
   - if [ "jruby" != "$TRAVIS_RUBY_VERSION"  ]; then gem install bundler --version "1.11.2"; fi