Replace string array literal with strings in IO.popen

This PR fixex https://github.com/grosser/parallel/issues/92

All the calls to popen have been changed to match a compatible signature for `IO.popen` as described in [1.8.7](http://ruby-doc.org/core-1.8.7/IO.html#method-c-popen), [1.9.3](http://ruby-doc.org/core-1.9.3/IO.html#method-c-popen), and [2.0.0](http://ruby-doc.org/core-2.0.0/IO.html#method-c-popen)

The fix has been tested in Mac OSX and Linux, in ree-1.8.7-2012.02, ruby-1.9.3-p448 and ruby-2.0.0-p247