Update .travis.yml

There is some weird error on jruby-head. 

```
$ bundle exec rspec spec/*
file:/home/travis/.rvm/rubies/jruby-head/lib/jruby.jar!/jruby/kernel/kernel.rb:25 warning: unsupported exec option: close_others
There was a Java::JavaLang::NullPointerException while loading prophet.gemspec: 
```