Allow fastcgi to use the DOCUMENT_ROOT from the web server

: We shouldn't use the current directory or the configured `SourceRoot` since the webserver could have many virtual hosts and always will send us a DOCUMENT_ROOT header.

The only scary part of the change is checking for `sourceRoot` instead of `getDocumentRoot` but I think it is right.

Differential Revision: D1108017
fix DOCUMENT_ROOT for fastcgi

We shouldn't have spcialized code in here for fastcgi, and it didn't even work.

Reviewed By: @markw65

Differential Revision: D1109072
bump version to 2.3.2