use jruby instead of jruby-18mode etc. at .travis.yml

git-svn-id: http://svn.redmine.org/redmine/trunk@12874 e93f8b46-1217-0410-a6f0-8f06a7374b81
allow jruby test failure on travis

SCM tests fail randomly due to IO.popen().

See:
https://github.com/jruby/jruby/issues/779

git-svn-id: http://svn.redmine.org/redmine/trunk@12875 e93f8b46-1217-0410-a6f0-8f06a7374b81