Adding timestamp to filename

[rfink]

This may not be the direction you guys are shooting for, so feel free to deny the pull request and close this out, although we had to modify this on our copy of rollbar.php to work in our environment. The current file name for the rollbar agent log is generated based on the process id of the current php process. In a high transaction environment, these could very well overlap. I'm proposing and adding a pull request in the new few moments that appends the timestamp to the process id to make sure a unique file name is generated. Thanks in advance for your consideration.