Adding curl error code 18 to retry behavior
Fix #437
Add progress compatibility for PHP 5.5

Long story short, there was a backwards incompatible change to the
CURLOPT_PROGRESSFUNCTION callback arguments.
Add tests.
Merge pull request #438 from Thinkscape/patch-1

Fix #437
Merge pull request #439 from adlawson/curl-progress

Fix cURL progress callback in PHP 5.5