Use raw Github URL for OS X plist

The cURL statement for https://github.com/gurgeh/selfspy/raw/master/com.github.gurgeh.selfspy.plist results in a file containing:

```<html><body>You are being <a href="https://raw.github.com/gurgeh/selfspy/master/com.github.gurgeh.selfspy.plist">redirected</a>.</body></html>```

Updated the statement to use the raw URL to avoid a redirect.