Update the downloading URL of source-sans-pro.

```
==> Downloading https://downloads.sourceforge.net/sourceforge/sourcesans.adobe/SourceSansPro_FontsOnly-2.010roman-1.065-italic.zip

curl: (22) The requested URL returned error: 404 Not Found
Error: Download failed on Cask 'font-source-sans-pro' with message: Download failed: https://downloads.sourceforge.net/sourceforge/sourcesans.adobe/SourceSansPro_FontsOnly-2.010roman-1.065-italic.zip
```