Fixes "Too many files open" regression on OS X

When we create a new `IPython.parallel` `Client` instance, we need to ensure it is `closed()`.  My previous PR #317 (specifically commit 5d3bcce6fa94cdb9a2997a1cf15594249c717b62) changed this behavior.

This PR restores the correct behavior and changes some `.cleanup()` calls in the testsuite to `.close()` instead.