Force JRuby not to select working directory as temporary directory

JRuby selects a temporary directory from the candidate directories,
which includes the ordinary /tmp and also the current working directory,
by checking if they are world writable or not. If /tmp is world writable
and the current working directory is not, the latter is chosen as a
temporary directory. This affects to behaviors of Tempfile and
Dir.mktmpdir. This is the reason that the spec context
"isolated enviroment" was actually not isolated on JRuby.

This is related to the following issue:
https://github.com/jruby/jruby/issues/405
Handle Rubinius dialect of NameError message
Fix "unable to modify frozen object" at splat expansion on Rubinius
Skip a broken spec on Rubinius

With rubinius 2.0.0.rc1 + rspec 2.13.1,
File.stub(:open).and_call_original causes SystemStackError.
Disallow failure of JRuby and Rubinius on Travis CI