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

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4012874
allow jruby test failure on travis

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

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

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4012875