Tag intermittent spec failures

This should improve the stability of the Travis build. Most of these failures are explained in #779 (and marked as such).  For the other two:

* `IO#close on an IO.popen stream sets $?` can been seen failing for no good reason [here](https://s3.amazonaws.com/archive.travis-ci.org/jobs/7496279/log.txt) (Note also that this is not a new tag; it simply fixes the tag so that it gets picked up by the build.  Copied from the [existing tag for 1.8](https://github.com/jruby/jruby/blob/dc3af39805c14195f3503e4d10861c6695f00ab5/spec/tags/1.8/ruby/core/io/close_tags.txt))
* `Process.kill accepts a signal name without the 'SIG' prefix` can be seen failing for no good reason [here](https://s3.amazonaws.com/archive.travis-ci.org/jobs/7570946/log.txt)