Update mongrel2 to version 1.9.1

Version 1.8.1 is outdated and has major bugs that are fixed in 1.9.1. 1.9.1 also has support for ZeroMQ, which obsoletes the patch required beforehand. Some versions of OSX wont build with `ENV.ldflags` by itself because the project depends on a library that doesn't export it's symbols, so the added ldflag allows it to build properly.